<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def behavior(f):
 """Decorator for declaring a function as behavior. Use as:: @behavior def fun(x, y, self, msg):
 And create or become this behavior by passing the two arguments `x` and `y`. """

    @wraps(f)
    def wrapper(*args):
        message = args[-1]
        if isinstance(message, MutableMapping):
            message = Message(message)
        f(*(args[:-1] + (message,)))
    return wrapper