def subscribe(self, callback, existing=True):
if callback in self.callbacks:<EOL><INDENT>return<EOL><DEDENT>self.callbacks.append(callback)<EOL>if not existing:<EOL><INDENT>return<EOL><DEDENT>for dist in self:<EOL><INDENT>callback(dist)<EOL><DEDENT>
Invoke `callback` for all distributions

        If `existing=True` (default),
        call on all existing ones, as well.