Add new example

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

examples/ex13.lean

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+
+Variable f {A : Type} (a b : A) : A
+Variable N : Type
+Variable n1 : N
+Variable n2 : N
+Set lean::pp::implicit true
+Show f n1 n2
+Show f (fun x : N -> N, x) (fun y : _, y)
+Variable EqNice {A : Type} (lhs rhs : A) : Bool
+Infix 50 === : EqNice
+Show n1 === n2
+Check f n1 n2
+Check Congr
+Show f n1 n2
+Theorem CongrI {A : Type u} {B : A → Type u} {f g : Π x : A, B x} {a b : A} (H1 : f = g) (H2 : a = b) : (f a) = (g b) :=
+        Congr A B f g a b H1 H2
+Theorem TransI {A : Type u} {a b c : A} (H1 : a = b) (H2 : b = c) : a = c :=
+        Trans A a b c H1 H2
+Theorem ReflI  {A : Type u} (a : A) : a = a :=
+        Refl A a
+Theorem SymmI  {A : Type u} {a b : A} (H : a = b) : b = a :=
+        Symm A a b H
+Theorem Conj1I {a b : Bool} (H : a && b) : a :=
+        Conjunct1 a b H
+Theorem Conj2I {a b : Bool} (H : a && b) : b :=
+        Conjunct2 a b H
+Variable a : N
+Variable b : N
+Variable c : N
+Variable g : N -> N
+Axiom H1 : a = b && b = c
+Theorem Pr : (g a) = (g c) :=
+    CongrI (ReflI g) (TransI (Conj1I H1) (Conj2I H1))
+Show Environment 1
+
+