code: def one_of(*args):
      "Verifies that only one of the arguments is not None"
      for i, arg in enumerate(args):
          if arg is not None:
              for argh in args[i+1:]:
                  if argh is not None:
                      raise OperationError("Too many parameters")
              else:
                  return
      raise OperationError("Insufficient parameters"f one_of(*args):
      "Verifies that only one of the arguments is not None"
      for i, arg in enumerate(args):
          if arg is not None:
              for argh in args[i+1:]:
                  if argh is not None:
                      raise OperationError("Too many parameters")
              else:
                  return
      raise OperationError("Insufficient parameters")
docs: Verifies that only one of the arguments is not None