<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def remove(self, fact):
 """Create an INVALID token and send it to all children."""

        token = Token.invalid(fact)
        MATCHER.debug("<BusNode> added %r", token)
        for child in self.children:
            child.callback(token)