def add_chars(w1, w2):
    '\n    Return a string containing the characters you need to add to w1 to get w2.\n\n    You may assume that w1 is a subsequence of w2.\n\n    >>> add_chars("owl", "howl")\n    \'h\'\n    >>> add_chars("want", "wanton")\n    \'on\'\n    >>> add_chars("rat", "radiate")\n    \'diae\'\n    >>> add_chars("a", "prepare")\n    \'prepre\'\n    >>> add_chars("resin", "recursion")\n    \'curo\'\n    >>> add_chars("fin", "effusion")\n    \'efuso\'\n    >>> add_chars("coy", "cacophony")\n    \'acphon\'\n    >>> from construct_check import check\n    >>> # ban iteration and sets\n    >>> check(LAB_SOURCE_FILE, \'add_chars\',\n    ...       [\'For\', \'While\', \'Set\', \'SetComp\']) # Must use recursion\n    True\n    '
    '*** YOUR CODE HERE ***'
Return a string containing the characters you need to add to w1 to get w2.

You may assume that w1 is a subsequence of w2.

>>> add_chars("owl", "howl")
'h'
>>> add_chars("want", "wanton")
'on'
>>> add_chars("rat", "radiate")
'diae'
>>> add_chars("a", "prepare")
'prepre'
>>> add_chars("resin", "recursion")
'curo'
>>> add_chars("fin", "effusion")
'efuso'
>>> add_chars("coy", "cacophony")
'acphon'
>>> from construct_check import check
>>> # ban iteration and sets
>>> check(LAB_SOURCE_FILE, 'add_chars',
...       ['For', 'While', 'Set', 'SetComp']) # Must use recursion
True
def add_chars(w1, w2):
    '\n    Return a string containing the characters you need to add to w1 to get w2.\n\n    You may assume that w1 is a subsequence of w2.\n\n    >>> add_chars("owl", "howl")\n    \'h\'\n    >>> add_chars("want", "wanton")\n    \'on\'\n    >>> add_chars("rat", "radiate")\n    \'diae\'\n    >>> add_chars("a", "prepare")\n    \'prepre\'\n    >>> add_chars("resin", "recursion")\n    \'curo\'\n    >>> add_chars("fin", "effusion")\n    \'efuso\'\n    >>> add_chars("coy", "cacophony")\n    \'acphon\'\n    >>> from construct_check import check\n    >>> # ban iteration and sets\n    >>> check(LAB_SOURCE_FILE, \'add_chars\',\n    ...       [\'For\', \'While\', \'Set\', \'SetComp\']) # Must use recursion\n    True\n    '
    '*** YOUR CODE HERE ***'