sympy/sympy — pull request #1626 by mrocklin (2012-10-30 13:35:19+00:00)
Title: add is_associative flag to Basic

I would like to be able to test if any Basic is associative. Previously this could have been done by checking

isinstance(x, AssocOp)

Unfortunately `AssocOp` inherits form `Expr` so this wouldn't make sense for matrices and sets.

This PR adds an `is_associative = None` flag to `Basic`