Associatively expand out nested arguments of the flat class.
    E.g.::

        >>> class Plus(Operation):
        ...     simplifications = [assoc, ]
        >>> Plus.create(1,Plus(2,3))
        Plus(1, 2, 3)

def assoc(cls, ops, kwargs):
    """Associatively expand out nested arguments of the flat class.
    E.g.::

        >>> class Plus(Operation):
        ...     simplifications = [assoc, ]
        >>> Plus.create(1,Plus(2,3))
        Plus(1, 2, 3)
    """
    expanded = [(o,) if not isinstance(o, cls) else o.operands for o in ops]
    return sum(expanded, ()), kwargs