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diff --git a/src/Inky/Decidable/Either.idr b/src/Inky/Decidable/Either.idr
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+module Inky.Decidable.Either
+
+import Data.These
+
+public export
+Union : Type -> Type -> Bool -> Type
+Union a b False = b
+Union a b True = a
+
+public export
+record LazyEither (0 a, b : Type) where
+  constructor Because
+  does : Bool
+  reason : Lazy (Union a b does)
+
+%name LazyEither p, q
+
+namespace Union
+  public export
+  map : {tag : Bool} -> (a -> c) -> (b -> d) -> Union a b tag -> Union c d tag
+  map {tag = False} f g x = g x
+  map {tag = True} f g x = f x
+
+  public export
+  not : {tag : Bool} -> Union a b tag -> Union b a (not tag)
+  not {tag = False} x = x
+  not {tag = True} x = x
+
+  public export
+  both :
+    {tag1 : Bool} -> {tag2 : Lazy Bool} ->
+    Union a b tag1 -> Lazy (Union c d tag2) ->
+    Union (a, c) (Either b d) (tag1 && tag2)
+  both {tag1 = True, tag2 = True} x y = (x, y)
+  both {tag1 = True, tag2 = False} x y = Right y
+  both {tag1 = False} x y = Left x
+
+  public export
+  first :
+    {tag1 : Bool} -> {tag2 : Lazy Bool} ->
+    Union a b tag1 -> Lazy (Union c d tag2) ->
+    Union (Either a c) (b, d) (tag1 || tag2)
+  first {tag1 = True} x y = Left x
+  first {tag1 = False, tag2 = True} x y = Right y
+  first {tag1 = False, tag2 = False} x y = (x, y)
+
+  public export
+  all :
+    {tag1, tag2 : Bool} ->
+    Union a b tag1 -> Union c d tag2 ->
+    Union (a, c) (These b d) (tag1 && tag2)
+  all {tag1 = True, tag2 = True} x y = (x, y)
+  all {tag1 = True, tag2 = False} x y = That y
+  all {tag1 = False, tag2 = True} x y = This x
+  all {tag1 = False, tag2 = False} x y = Both x y
+
+  public export
+  any :
+    {tag1, tag2 : Bool} ->
+    Union a b tag1 -> Union c d tag2 ->
+    Union (These a c) (b, d) (tag1 || tag2)
+  any {tag1 = True, tag2 = True} x y = Both x y
+  any {tag1 = True, tag2 = False} x y = This x
+  any {tag1 = False, tag2 = True} x y = That y
+  any {tag1 = False, tag2 = False} x y = (x, y)
+
+public export
+map : (a -> c) -> (b -> d) -> LazyEither a b -> LazyEither c d
+map f g p = p.does `Because` Union.map f g p.reason
+
+public export
+not : LazyEither a b -> LazyEither b a
+not p = not p.does `Because` Union.not p.reason
+
+public export
+both : LazyEither a b -> Lazy (LazyEither c d) -> LazyEither (a, c) (Either b d)
+both p q = p.does && q.does `Because` Union.both p.reason q.reason
+
+public export
+first : LazyEither a b -> Lazy (LazyEither c d) -> LazyEither (Either a c) (b, d)
+first p q = p.does || q.does `Because` Union.first p.reason q.reason
+
+public export
+all : LazyEither a b -> Lazy (LazyEither c d) -> LazyEither (a, c) (These b d)
+all p q = p.does && q.does `Because` Union.all p.reason q.reason
+
+public export
+any : LazyEither a b -> Lazy (LazyEither c d) -> LazyEither (These a c) (b, d)
+any p q = p.does || q.does `Because` Union.any p.reason q.reason
+
+export autobind infixr 0 >=>
+
+public export
+andThen :
+  {0 p, q : a -> Type} ->
+  LazyEither a b -> ((x : a) -> LazyEither (p x) (q x)) ->
+  LazyEither (x ** p x) (Either b (x ** q x))
+andThen (True `Because` x) f = map (\px => (x ** px)) (\qx => Right (x ** qx)) (f x)
+andThen (False `Because` y) f = False `Because` (Left y)
+
+public export
+(>=>) :
+  {0 p, q : a -> Type} ->
+  LazyEither a b -> ((x : a) -> LazyEither (p x) (q x)) ->
+  LazyEither (x ** p x) (Either b (x ** q x))
+(>=>) = andThen