#!/usr/local/bin/cadabra2
ex := A_{m n} B_{m n};
type(ex);
A_{m n}::Symmetric;
B_{m n}::AntiSymmetric.
canonicalise(ex);
ex  := A_{m n} B_{m n};
rule:= A_{m n} -> A_{m n} + C_{m n};
for i in range(3):
	substitute(ex, rule);