docs: def filter_neutral(cls, ops, kwargs):
      """Remove occurrences of a neutral element from the argument/operand list,
      if that list has at least two elements.  To use this, one must also specify
      a neutral element, which can be anything that allows for an equality check
      with each argument.  E.g.::
  
          >>> class X(Operation):
          ...     _neutral_element = 1
          ...     simplifications = [filter_neutral, ]
          >>> X.create(2,1,3,1)
          X(2, 3)
      """
      c_n = cls._neutral_element
      if len(ops) == 0:
          return c_n
      fops = [op for op in ops if c_n != op]  # op != c_n does NOT work
      if len(fops) > 1:
          return fops, kwargs
      elif len(fops) == 1:
          # the remaining operand is the single non-trivial one
          return fops[0]
      else:
          # the original list of operands consists only of neutral elements
          return ops[0]