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")
Verifies that only one of the arguments is not None