<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _FlattenToCallback(tokens, callback):
 """Takes a nested list structure and flattens it. ['a', ['b', 'c']] -> callback('a'); callback('b'); callback('c'); """

    for t in tokens:
        if isinstance(t, six.string_types):
            callback(t)
        else:
            _FlattenToCallback(t, callback)