docs: 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