Write about equivalence relations

1 files changed, 41 insertions, 2 deletions

diff --git a/doc/Tutorial.html b/doc/Tutorial.html
index ae77959..5336f7a 100644
--- a/doc/Tutorial.html
+++ b/doc/Tutorial.html
@@ -44,5 +44,44 @@
 <li><p>constructing types with an equality relation that’s better behaved than Idris’s built-in <code>Equal</code> type.</p></li>
 <li><p>types with an equality relation that carries additional information</p></li>
 </ul>
-<h2 id="basic-interface">Basic interface</h2>
-<p><code class="IdrisCode"> <br /> </code></p>
+<p>If you want to see the source-code behind this tutorial, check the <a href="sources/Tutorial.md">source-code</a> out.</p>
+<h2 id="equivalence-relations">Equivalence relations</h2>
+<p>A <em>relation</em> over a type <code>ty</code> in Idris is any two-argument type-valued function:</p>
+<pre><code>namespace Control.Relation
+  Rel : Type -&gt; Type
+  Rel ty = ty -&gt; ty -&gt; Type</code></pre>
+<p>This definition and its associated interfaces ship with idris’s standard library. Given a relation <code>rel : Rel ty</code> and <code>x,y : ty</code>, we can form <code>x `rel` y : Type</code>: the type of ways in which <code>x</code> and <code>y</code> can be related.</p>
+<p>For example, two lists <em>overlap</em> when they have a common element: <code class="IdrisCode"> <span class="IdrisKeyword">record</span> <span class="IdrisType">Overlap</span> <span class="IdrisKeyword">{0</span> <span class="IdrisBound">a</span> <span class="IdrisKeyword">:</span> <span class="IdrisType">Type</span><span class="IdrisKeyword">}</span> <span class="IdrisKeyword">(</span><span class="IdrisBound">xs</span><span class="IdrisKeyword">,</span><span class="IdrisBound">ys</span> <span class="IdrisKeyword">:</span> <span class="IdrisType">List</span> <span class="IdrisBound">a</span><span class="IdrisKeyword">)</span> <span class="IdrisKeyword">where</span><br />   constructor <span class="IdrisData">Overlapping</span><br />   <span class="IdrisFunction">common</span> <span class="IdrisKeyword">:</span> <span class="IdrisBound">a</span><br />   <span class="IdrisFunction">lhsPos</span> <span class="IdrisKeyword">:</span> <span class="IdrisBound">common</span> <span class="IdrisType"><code>Elem</code></span> <span class="IdrisBound">xs</span><br />   <span class="IdrisFunction">rhsPos</span> <span class="IdrisKeyword">:</span> <span class="IdrisBound">common</span> <span class="IdrisType"><code>Elem</code></span> <span class="IdrisBound">ys</span><br /> <br /> </code> Lists can overlap in exactly one position: <code class="IdrisCode"> <span class="IdrisFunction">Ex1</span> <span class="IdrisKeyword">:</span> <span class="IdrisType">Overlap</span> <span class="IdrisData">[1,2,3]</span> <span class="IdrisData">[6,7,2,8]</span><br /> <span class="IdrisFunction">Ex1</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">Overlapping</span><br />   <span class="IdrisKeyword">{</span> <span class="IdrisBound">common</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">2</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">lhsPos</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">There</span> <span class="IdrisData">Here</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">rhsPos</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">There</span> <span class="IdrisKeyword">(</span><span class="IdrisData">There</span> <span class="IdrisData">Here</span><span class="IdrisKeyword">)</span><br />   <span class="IdrisKeyword">}</span><br /> </code> But they can overlap in several ways: <code class="IdrisCode"> <span class="IdrisFunction">Ex2a</span> <span class="IdrisKeyword">,</span><br /> <span class="IdrisFunction">Ex2b</span> <span class="IdrisKeyword">,</span><br /> <span class="IdrisFunction">Ex2c</span> <span class="IdrisKeyword">:</span> <span class="IdrisType">Overlap</span> <span class="IdrisData">[1,2,3]</span> <span class="IdrisData">[2,3,2]</span><br /> <span class="IdrisFunction">Ex2a</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">Overlapping</span><br />   <span class="IdrisKeyword">{</span> <span class="IdrisBound">common</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">3</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">lhsPos</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">There</span> <span class="IdrisKeyword">(</span><span class="IdrisData">There</span> <span class="IdrisData">Here</span><span class="IdrisKeyword">)</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">rhsPos</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">There</span> <span class="IdrisData">Here</span><br />   <span class="IdrisKeyword">}</span><br /> <span class="IdrisFunction">Ex2b</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">Overlapping</span><br />   <span class="IdrisKeyword">{</span> <span class="IdrisBound">common</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">2</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">lhsPos</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">There</span> <span class="IdrisData">Here</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">rhsPos</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">Here</span><br />   <span class="IdrisKeyword">}</span><br /> <span class="IdrisFunction">Ex2c</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">Overlapping</span><br />   <span class="IdrisKeyword">{</span> <span class="IdrisBound">common</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">2</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">lhsPos</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">There</span> <span class="IdrisData">Here</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">rhsPos</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">There</span> <span class="IdrisKeyword">(</span><span class="IdrisData">There</span> <span class="IdrisData">Here</span><span class="IdrisKeyword">)</span><br />   <span class="IdrisKeyword">}</span><br /> </code> We can think of a relation <code>rel : Rel ty</code> as the type of edges in a directed graph between vertices in <code>ty</code>:</p>
+<ul>
+<li><p>edges have a direction: the type <code>rel x y</code> is different to <code>rel y x</code></p></li>
+<li><p>multiple different edges between the same vertices <code>e1, e2 : rel x y</code></p></li>
+<li><p>self-loops between the same vertex are allowed <code>loop : rel x x</code>.</p></li>
+</ul>
+<p>An <em>equivalence relation</em> is a relation that’s:</p>
+<ul>
+<li><p><em>reflexive</em>: we guarantee a specific way in which every element is related to itself;</p></li>
+<li><p><em>symmetric</em>: we can reverse an edge between two edges; and</p></li>
+<li><p><em>transitive</em>: we can compose paths of related elements into a single edge.</p></li>
+</ul>
+<p><code class="IdrisCode"> <span class="IdrisKeyword">record</span> <span class="IdrisType">Equivalence</span> <span class="IdrisKeyword">(</span><span class="IdrisBound">A</span> <span class="IdrisKeyword">:</span> <span class="IdrisType">Type</span><span class="IdrisKeyword">)</span> <span class="IdrisKeyword">where</span><br />   constructor <span class="IdrisData">MkEquivalence</span><br />   <span class="IdrisKeyword">0</span> <span class="IdrisFunction">relation</span><span class="IdrisKeyword">:</span> <span class="IdrisFunction">Rel</span> <span class="IdrisBound">A</span><br />   <span class="IdrisFunction">reflexive</span> <span class="IdrisKeyword">:</span> <span class="IdrisKeyword">(</span><span class="IdrisBound">x</span>       <span class="IdrisKeyword">:</span> <span class="IdrisBound">A</span><span class="IdrisKeyword">)</span> <span class="IdrisKeyword">-&gt;</span> <span class="IdrisBound">relation</span> <span class="IdrisBound">x</span> <span class="IdrisBound">x</span><br />   <span class="IdrisFunction">symmetric</span> <span class="IdrisKeyword">:</span> <span class="IdrisKeyword">(</span><span class="IdrisBound">x</span><span class="IdrisKeyword">,</span> <span class="IdrisBound">y</span>    <span class="IdrisKeyword">:</span> <span class="IdrisBound">A</span><span class="IdrisKeyword">)</span> <span class="IdrisKeyword">-&gt;</span> <span class="IdrisBound">relation</span> <span class="IdrisBound">x</span> <span class="IdrisBound">y</span> <span class="IdrisKeyword">-&gt;</span> <span class="IdrisBound">relation</span> <span class="IdrisBound">y</span> <span class="IdrisBound">x</span><br />   <span class="IdrisFunction">transitive</span><span class="IdrisKeyword">:</span> <span class="IdrisKeyword">(</span><span class="IdrisBound">x</span><span class="IdrisKeyword">,</span> <span class="IdrisBound">y</span><span class="IdrisKeyword">,</span> <span class="IdrisBound">z</span> <span class="IdrisKeyword">:</span> <span class="IdrisBound">A</span><span class="IdrisKeyword">)</span> <span class="IdrisKeyword">-&gt;</span> <span class="IdrisBound">relation</span> <span class="IdrisBound">x</span> <span class="IdrisBound">y</span> <span class="IdrisKeyword">-&gt;</span> <span class="IdrisBound">relation</span> <span class="IdrisBound">y</span> <span class="IdrisBound">z</span><br />                             <span class="IdrisKeyword">-&gt;</span> <span class="IdrisBound">relation</span> <span class="IdrisBound">x</span> <span class="IdrisBound">z</span><br /> </code></p>
+<p>We equip the built-in relation <code>Equal</code> with the structure of an equivalence relation, using the constructor <code>Refl</code> and the stdlib functions <code>sym</code>, and <code>trans</code>: <code class="IdrisCode"> <span class="IdrisFunction">EqualityEquivalence</span> <span class="IdrisKeyword">:</span> <span class="IdrisType">Equivalence</span> <span class="IdrisBound">a</span><br /> <span class="IdrisFunction">EqualityEquivalence</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">MkEquivalence</span><br />   <span class="IdrisKeyword">{</span> <span class="IdrisBound">relation</span> <span class="IdrisKeyword">=</span> <span class="IdrisFunction">(===)</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">reflexive</span> <span class="IdrisKeyword">=</span> <span class="IdrisKeyword">&lt;/span&gt;<span class="IdrisBound">x</span> <span class="IdrisKeyword">=&gt;</span> <span class="IdrisData">Refl</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">symmetric</span> <span class="IdrisKeyword">=</span> <span class="IdrisKeyword">&lt;/span&gt;<span class="IdrisBound">x</span><span class="IdrisKeyword">,</span><span class="IdrisBound">y</span><span class="IdrisKeyword">,</span><span class="IdrisBound">x_eq_y</span> <span class="IdrisKeyword">=&gt;</span> <span class="IdrisFunction">sym</span> <span class="IdrisBound">x_eq_y</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">transitive</span> <span class="IdrisKeyword">=</span> <span class="IdrisKeyword">&lt;/span&gt;<span class="IdrisBound">x</span><span class="IdrisKeyword">,</span><span class="IdrisBound">y</span><span class="IdrisKeyword">,</span><span class="IdrisBound">z</span><span class="IdrisKeyword">,</span><span class="IdrisBound">x_eq_y</span><span class="IdrisKeyword">,</span><span class="IdrisBound">y_eq_z</span> <span class="IdrisKeyword">=&gt;</span> <span class="IdrisFunction">trans</span> <span class="IdrisBound">x_eq_y</span> <span class="IdrisBound">y_eq_z</span><br />   <span class="IdrisKeyword">}</span><br /> </code></p>
+<p>We’ll use the following relation on pairs of natural numbers as a running example. We can represent an integer as the difference between a pair of natural numbers: <code class="IdrisCode"> <span class="IdrisKeyword">infix</span> <span class="IdrisKeyword">8</span> .-.<br /> <br /> <span class="IdrisKeyword">record</span> <span class="IdrisType">INT</span> <span class="IdrisKeyword">where</span><br />   constructor <span class="IdrisData">(.-.)</span><br />   <span class="IdrisFunction">pos</span><span class="IdrisKeyword">,</span> <span class="IdrisFunction">neg</span> <span class="IdrisKeyword">:</span> <span class="IdrisType">Nat</span><br /> <br /> <span class="IdrisKeyword">record</span> <span class="IdrisType">SameDiff</span> <span class="IdrisKeyword">(</span><span class="IdrisBound">x</span><span class="IdrisKeyword">,</span> <span class="IdrisBound">y</span> <span class="IdrisKeyword">:</span> <span class="IdrisType">INT</span><span class="IdrisKeyword">)</span> <span class="IdrisKeyword">where</span><br />   constructor <span class="IdrisData">Check</span><br />   <span class="IdrisFunction">same</span> <span class="IdrisKeyword">:</span> <span class="IdrisKeyword">(</span><span class="IdrisBound">x</span><span class="IdrisFunction">.pos + </span><span class="IdrisBound">y</span><span class="IdrisFunction">.neg === </span><span class="IdrisBound">y</span><span class="IdrisFunction">.pos + </span><span class="IdrisBound">x</span><span class="IdrisFunction">.neg</span><span class="IdrisKeyword">)</span><br /> </code> The <code>SameDiff x y</code> relation is equivalent to mathematical equation that states that the difference between the positive and negative parts is identical: <br /><span class="math display"><em>x</em><sub><em>p</em><em>o</em><em>s</em></sub> − <em>x</em><sub><em>n</em><em>e</em><em>g</em></sub> = <em>y</em><sub><em>p</em><em>o</em><em>s</em></sub> − <em>y</em><sub><em>n</em><em>e</em><em>g</em></sub></span><br /> But, unlike the last equation which requires us to define integers and subtraction, its equivalent <code>(.same)</code> is expressed using only addition, and so addition on <code>Nat</code> is enough.</p>
+<p>The relation <code>SameDiff</code> is an equivalence relation. The proofs are straightforward, and a good opportunity to practice Idris’s equational reasoning combinators from <code>Syntax.PreorderReasoning</code>: <code class="IdrisCode"> <span class="IdrisFunction">SameDiffEquivalence</span> <span class="IdrisKeyword">:</span> <span class="IdrisType">Equivalence</span> <span class="IdrisType">INT</span><br /> <span class="IdrisFunction">SameDiffEquivalence</span> <span class="IdrisKeyword">=</span> <span class="IdrisData">MkEquivalence</span><br />   <span class="IdrisKeyword">{</span> <span class="IdrisBound">relation</span> <span class="IdrisKeyword">=</span> <span class="IdrisType">SameDiff</span><br />   <span class="IdrisKeyword">,</span> <span class="IdrisBound">reflexive</span> <span class="IdrisKeyword">=</span> <span class="IdrisKeyword">&lt;/span&gt;<span class="IdrisBound">x</span> <span class="IdrisKeyword">=&gt;</span> <span class="IdrisData">Check</span> <span class="math inline">$&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;Calc&lt;/span&gt;&amp;nbsp;$</span><br />       <span class="IdrisData">|~</span> <span class="IdrisBound">x</span><span class="IdrisFunction">.pos</span> <span class="IdrisFunction">+</span> <span class="IdrisBound">x</span><span class="IdrisFunction">.neg</span><br />       <span class="IdrisData">~~</span> <span class="IdrisBound">x</span><span class="IdrisFunction">.pos</span> <span class="IdrisFunction">+</span> <span class="IdrisBound">x</span><span class="IdrisFunction">.neg</span> <span class="IdrisData">…</span><span class="IdrisKeyword">(</span><span class="IdrisData">Refl</span><span class="IdrisKeyword">)</span><br /> </code> This equational proof represents the single-step equational proof:</p>
+<p>&quot;Calculate:</p>
+<ol type="1">
+<li><span class="math inline"><em>x</em><sub><em>p</em><em>o</em><em>s</em></sub> + <em>x</em><sub><em>n</em><em>e</em><em>g</em></sub></span></li>
+<li><span class="math inline"> = <em>x</em><sub><em>p</em><em>o</em><em>s</em></sub> + <em>x</em><sub><em>n</em><em>e</em><em>g</em></sub></span> (by reflexivity)&quot;</li>
+</ol>
+<p>The mnemonic behind the ASCII-art is that the first step in the proof starts with a logical-judgement symbol <span class="math inline">⊢</span>, each step continues with an equality sign <span class="math inline">=</span>, and justified by a thought bubble <code>(...)</code>.</p>
+<p><code class="IdrisCode">   <span class="IdrisKeyword">,</span> <span class="IdrisBound">symmetric</span> <span class="IdrisKeyword">=</span> <span class="IdrisKeyword">&lt;/span&gt;<span class="IdrisBound">x</span><span class="IdrisKeyword">,</span><span class="IdrisBound">y</span><span class="IdrisKeyword">,</span><span class="IdrisBound">x_eq_y</span> <span class="IdrisKeyword">=&gt;</span> <span class="IdrisData">Check</span> <span class="math inline">$&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;Calc&lt;/span&gt;&amp;nbsp;$</span><br />       <span class="IdrisData">|~</span> <span class="IdrisBound">y</span><span class="IdrisFunction">.pos</span> <span class="IdrisFunction">+</span> <span class="IdrisBound">x</span><span class="IdrisFunction">.neg</span><br />       <span class="IdrisData">~~</span> <span class="IdrisBound">x</span><span class="IdrisFunction">.pos</span> <span class="IdrisFunction">+</span> <span class="IdrisBound">y</span><span class="IdrisFunction">.neg</span> <span class="IdrisFunction">..&lt;</span><span class="IdrisKeyword">(</span><span class="IdrisBound">x_eq_y</span><span class="IdrisFunction">.same</span><span class="IdrisKeyword">)</span><br /> </code> In this proof, we were given the proof <code>x_eq_y.same : x.pos + y.neg = y.pos + x.neg</code> and so we appealed to the symmetric equation. The mnemonic here is that the last bubble in the thought bubble <code>(...)</code> is replace with a left-pointing arrow, reversing the reasoning step. <code class="IdrisCode">   <span class="IdrisKeyword">,</span> <span class="IdrisBound">transitive</span> <span class="IdrisKeyword">=</span> <span class="IdrisKeyword">&lt;/span&gt;<span class="IdrisBound">x</span><span class="IdrisKeyword">,</span><span class="IdrisBound">y</span><span class="IdrisKeyword">,</span><span class="IdrisBound">z</span><span class="IdrisKeyword">,</span><span class="IdrisBound">x_eq_y</span><span class="IdrisKeyword">,</span><span class="IdrisBound">y_eq_z</span> <span class="IdrisKeyword">=&gt;</span> <span class="IdrisData">Check</span> <span class="math inline">$&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;plusRightCancel&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisKeyword&quot;&gt;\_&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisKeyword&quot;&gt;\_&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisBound&quot;&gt;y&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;.pos&lt;/span&gt;&lt;br /&gt; &amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;$</span> <span class="IdrisFunction">Calc</span> <span class="math inline">$&lt;br /&gt; &amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;span class=&quot;IdrisData&quot;&gt;|~&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisBound&quot;&gt;x&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;.pos&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;+&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisBound&quot;&gt;z&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;.neg&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;+&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisBound&quot;&gt;y&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;.pos&lt;/span&gt;&lt;br /&gt; &amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;span class=&quot;IdrisData&quot;&gt;~~&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisBound&quot;&gt;x&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;.pos&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;+&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisKeyword&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;IdrisBound&quot;&gt;y&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;.pos&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;+&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisBound&quot;&gt;z&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;.neg&lt;/span&gt;&lt;span class=&quot;IdrisKeyword&quot;&gt;)&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisData&quot;&gt;...&lt;/span&gt;&lt;span class=&quot;IdrisKeyword&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;solve&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisData&quot;&gt;3&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;Monoid.Commutative.Free.Free&lt;/span&gt;&lt;br /&gt; &amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;span class=&quot;IdrisKeyword&quot;&gt;{&lt;/span&gt;&lt;span class=&quot;IdrisBound&quot;&gt;a&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisKeyword&quot;&gt;=&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;Nat.Additive&lt;/span&gt;&lt;span class=&quot;IdrisKeyword&quot;&gt;}&lt;/span&gt;&amp;nbsp;$</span><br />                                      <span class="IdrisKeyword">(</span><span class="IdrisFunction">X</span> <span class="IdrisData">0</span> <span class="IdrisFunction">.+.</span> <span class="IdrisFunction">X</span> <span class="IdrisData">1</span><span class="IdrisKeyword">)</span> <span class="IdrisFunction">.+.</span> <span class="IdrisFunction">X</span> <span class="IdrisData">2</span><br />                                   <span class="IdrisFunction">=-=</span> <span class="IdrisFunction">X</span> <span class="IdrisData">0</span> <span class="IdrisFunction">.+.</span> <span class="IdrisKeyword">(</span><span class="IdrisFunction">X</span> <span class="IdrisData">2</span> <span class="IdrisFunction">.+.</span> <span class="IdrisFunction">X</span> <span class="IdrisData">1</span><span class="IdrisKeyword">))</span><br />       <span class="IdrisData">~~</span> <span class="IdrisBound">x</span><span class="IdrisFunction">.pos</span> <span class="IdrisFunction">+</span> <span class="IdrisKeyword">(</span><span class="IdrisBound">z</span><span class="IdrisFunction">.pos</span> <span class="IdrisFunction">+</span> <span class="IdrisBound">y</span><span class="IdrisFunction">.neg</span><span class="IdrisKeyword">)</span> <span class="IdrisData">…</span><span class="IdrisKeyword">(</span><span class="IdrisFunction">cong</span> <span class="IdrisKeyword">(</span><span class="IdrisBound">x</span><span class="IdrisFunction">.pos</span> <span class="IdrisFunction">+</span><span class="IdrisKeyword">)</span> <span class="math inline">$&amp;nbsp;&lt;span class=&quot;IdrisBound&quot;&gt;y\_eq\_z&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;.same&lt;/span&gt;&lt;span class=&quot;IdrisKeyword&quot;&gt;)&lt;/span&gt;&lt;br /&gt; &amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;span class=&quot;IdrisData&quot;&gt;~~&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisKeyword&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;IdrisBound&quot;&gt;x&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;.pos&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;+&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisBound&quot;&gt;y&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;.neg&lt;/span&gt;&lt;span class=&quot;IdrisKeyword&quot;&gt;)&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;+&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisBound&quot;&gt;z&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;.pos&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisData&quot;&gt;...&lt;/span&gt;&lt;span class=&quot;IdrisKeyword&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;IdrisFunction&quot;&gt;solve&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisData&quot;&gt;3&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;Monoid.Commutative.Free.Free&lt;/span&gt;&lt;br /&gt; &amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;span class=&quot;IdrisKeyword&quot;&gt;{&lt;/span&gt;&lt;span class=&quot;IdrisBound&quot;&gt;a&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisKeyword&quot;&gt;=&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;IdrisFunction&quot;&gt;Nat.Additive&lt;/span&gt;&lt;span class=&quot;IdrisKeyword&quot;&gt;}&lt;/span&gt;&amp;nbsp;$</span><br />                                      <span class="IdrisFunction">X</span> <span class="IdrisData">0</span> <span class="IdrisFunction">.+.</span> <span class="IdrisKeyword">(</span><span class="IdrisFunction">X</span> <span class="IdrisData">1</span> <span class="IdrisFunction">.+.</span> <span class="IdrisFunction">X</span> <span class="IdrisData">2</span><span class="IdrisKeyword">)</span><br />                                  <span class="IdrisFunction">=-=</span> <span class="IdrisKeyword">(</span><span class="IdrisFunction">X</span> <span class="IdrisData">0</span> <span class="IdrisFunction">.+.</span> <span class="IdrisFunction">X</span> <span class="IdrisData">2</span><span class="IdrisKeyword">)</span> <span class="IdrisFunction">.+.</span> <span class="IdrisFunction">X</span> <span class="IdrisData">1</span><span class="IdrisKeyword">)</span><br />       <span class="IdrisData">~~</span> <span class="IdrisKeyword">(</span><span class="IdrisBound">y</span><span class="IdrisFunction">.pos</span> <span class="IdrisFunction">+</span> <span class="IdrisBound">x</span><span class="IdrisFunction">.neg</span><span class="IdrisKeyword">)</span> <span class="IdrisFunction">+</span> <span class="IdrisBound">z</span><span class="IdrisFunction">.pos</span> <span class="IdrisData">…</span><span class="IdrisKeyword">(</span><span class="IdrisFunction">cong</span> <span class="IdrisKeyword">(</span><span class="IdrisFunction">+</span> <span class="IdrisBound">z</span><span class="IdrisFunction">.pos</span><span class="IdrisKeyword">)</span> ?h2<span class="IdrisKeyword">)</span><br />       <span class="IdrisData">~~</span> <span class="IdrisBound">z</span><span class="IdrisFunction">.pos</span> <span class="IdrisFunction">+</span> <span class="IdrisBound">x</span><span class="IdrisFunction">.neg</span> <span class="IdrisFunction">+</span> <span class="IdrisBound">y</span><span class="IdrisFunction">.pos</span>   <span class="IdrisData">…</span><span class="IdrisKeyword">(</span><span class="IdrisFunction">solve</span> <span class="IdrisData">3</span> <span class="IdrisFunction">Monoid.Commutative.Free.Free</span><br />                                      <span class="IdrisKeyword">{</span><span class="IdrisBound">a</span> <span class="IdrisKeyword">=</span> <span class="IdrisFunction">Nat.Additive</span><span class="IdrisKeyword">}</span> $<br />                                      <span class="IdrisKeyword">(</span><span class="IdrisFunction">X</span> <span class="IdrisData">0</span> <span class="IdrisFunction">.+.</span> <span class="IdrisFunction">X</span> <span class="IdrisData">1</span><span class="IdrisKeyword">)</span> <span class="IdrisFunction">.+.</span> <span class="IdrisFunction">X</span> <span class="IdrisData">2</span><br />                                  <span class="IdrisFunction">=-=</span> <span class="IdrisKeyword">(</span><span class="IdrisFunction">X</span> <span class="IdrisData">2</span> <span class="IdrisFunction">.+.</span> <span class="IdrisFunction">X</span> <span class="IdrisData">1</span><span class="IdrisKeyword">)</span> <span class="IdrisFunction">.+.</span> <span class="IdrisFunction">X</span> <span class="IdrisData">0</span><span class="IdrisKeyword">)</span><br />   <span class="IdrisKeyword">}</span><br /> </code> This proof is a lot more involved:</p>
+<ol type="1">
+<li>We appeal to the cancellation property of addition: <span class="math inline"><em>a</em> + <em>c</em> = <em>b</em> + <em>c</em> ⇒ <em>a</em> = <em>b</em></span></li>
+<li>We rearrange the term, bringing the appropriate part of <code>y</code> into contact with the appropriate part of <code>z</code> and <code>x</code> to transform the term.</li>
+</ol>
+<p>Here we use the idris library <a href="http://www.github.com/frex-project/idris-frex"><code>Frex</code></a> that can perform such routine rearrangements for common algebraic structures. In this case, we use the commutative monoid simplifier from <code>Frex</code>. If you want to read more about <code>Frex</code>, check the <a href="https://www.denotational.co.uk/drafts/allais-brady-corbyn-kammar-yallop-frex-dependently-typed-algebraic-simplification.pdf">paper</a> out.</p>
+<p>Idris’s <code>Control.Relation</code> defines interfaces for properties like reflexivity and transitivity. While the setoid package doesn’t use them, we’ll use them in a few examples.</p>
+<p>The <code>Overlap</code> relation from Examples 1 and 2 is symmetric: <code class="IdrisCode"> Sy<span class="IdrisFunction">mmetric (</span>L<span class="IdrisBound">ist a) Overlap</span> <span class="IdrisKeyword">w</span>h<span class="IdrisData">ere</span><br />   sy<span class="IdrisKeyword">m</span>m<span class="IdrisBound">etric </span>x<span class="IdrisKeyword">s</span>_<span class="IdrisBound">overlaps_ys = </span><span class="IdrisFunction">Overlap</span>ping<br />     <span class="IdrisKeyword">{</span> <span class="IdrisBound">common</span> <span class="IdrisKeyword">=</span> <span class="IdrisBound">xs_overlaps_ys</span><span class="IdrisFunction">.common</span><br />     <span class="IdrisKeyword">,</span> <span class="IdrisBound">lhsPos</span> <span class="IdrisKeyword">=</span> <span class="IdrisBound">xs_overlaps_ys</span><span class="IdrisFunction">.rhsPos</span><br />     <span class="IdrisKeyword">,</span> rhsPos = xs_overlaps_ys.lhsPos<br />     }<br /> </code> However, <code>Overlap</code> is neither reflexive nor transitive:</p>
+<ul>
+<li><p>The empty list doesn’t overlap with itself: <code class="IdrisCode"> <span class="IdrisFunction">Ex3</span> <span class="IdrisBound">: Not (Overlap [</span>]<span class="IdrisKeyword"> </span>[<span class="IdrisKeyword">])</span><br /> Ex<span class="IdrisKeyword">3</span> <span class="IdrisKeyword">nil_overla</span>ps_nil = case nil_overlaps_nil.lhsPos of<br />   _ impossible<br /> </code></p></li>
+<li><p>Two lists may overlap with a middle list, but on different elements. For example: <code class="IdrisCode"> Ex4 : <span class="IdrisType">(</span> <span class="IdrisType">Overlap</span> <span class="IdrisData">[1] [</span>1<span class="IdrisData">,2]</span><br />       <span class="IdrisType">,</span> <span class="IdrisFunction">Ove</span>r<span class="IdrisKeyword">l</span><span class="IdrisType">ap [1,2</span>]<span class="IdrisData"> [2</span>]<br /> <span class="IdrisFunction">   </span> <span class="IdrisKeyword"> </span> , Not (Overlap [1] [2]))<br /> Ex<span class="IdrisKeyword">4</span> <span class="IdrisData">=</span><br />   <span class="IdrisData">(</span> <span class="IdrisData">Overlapping</span> <span class="IdrisData">1</span> <span class="IdrisKeyword">H</span><span class="IdrisData">ere H</span>e<span class="IdrisData">re</span><br />   <span class="IdrisData">,</span> <span class="IdrisKeyword">O</span>v<span class="IdrisBound">erlapping 2 (The</span>r<span class="IdrisKeyword">e </span>H<span class="IdrisKeyword">ere)</span> <span class="IdrisBound">Here</span><br />   , &lt;span class=“IdrisData”&gt; one_</span>o<span class="IdrisKeyword">v</span>e<span class="IdrisKeyword">rlaps_two </span>=&gt; case one_overlaps_two.lhsPos of<br />   <span class="IdrisKeyword"> </span>  There _ impossible<br />   )<br /> </code> The outer lists agree on <code>1</code> and <code>2</code>, respectively, but they can’t overlap on on the first element of either, which exhausts all possibilities of overlap.</p></li>
+</ul>