test(tests/lean): matrix multiplication example

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

tests/lean/matrix.lean

@@ -0,0 +1,18 @@
+variable matrix : Nat → Nat → Type
+variable mul {m n p : Nat} : matrix n m → matrix m p → matrix n p
+infixl 70 * : mul
+axiom mul_assoc {m n p o : Nat} (M : matrix n m) (N : matrix m p) (P : matrix p o) :
+      M * (N * P) = (M * N) * P
+add_rewrite mul_assoc
+
+-- Create an example
+variable m1 : matrix 2 3
+variable m2 : matrix 3 4
+variable m3 : matrix 4 2
+variable m4 : matrix 2 6
+
+(*
+local t = parse_lean("m1 * (m2 * (m3 * m4))")
+print("before simp: " .. tostring(t))
+print("after simp:  " .. tostring(simplify(t)))
+*)