loose checking if s is a pytables-acceptable expression

def maybe_expression(s):
    """ loose checking if s is a pytables-acceptable expression """
    if not isinstance(s, str):
        return False
    ops = ExprVisitor.binary_ops + ExprVisitor.unary_ops + ('=',)

    # make sure we have an op at least
    return any(op in s for op in ops)