<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def doit( self, classes=None, recursive=True, indices=None, max_terms=None, **kwargs):
 """Write out the indexed sum explicitly If `classes` is None or :class:`IndexedSum` is in `classes`, (partially) write out the indexed sum in to an explicit sum of terms. If `recursive` is True, write out each of the new sum's summands by calling its :meth:`doit` method. Args: classes (None or list):
 see :meth:`.Expression.doit` recursive (bool):
 see :meth:`.Expression.doit` indices (list):
 List of :class:`IdxSym` indices for which the sum should be expanded. If `indices` is a subset of the indices over which the sum runs, it will be partially expanded. If not given, expand the sum completely max_terms (int):
 Number of terms after which to truncate the sum. This is particularly useful for infinite sums. If not given, expand all terms of the sum. Cannot be combined with `indices` kwargs: keyword arguments for recursive calls to :meth:`doit`. See :meth:`.Expression.doit` """

        return super().doit(
            classes, recursive, indices=indices, max_terms=max_terms, **kwargs)