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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def list_structures(filename, treename=None):
 """Get a dictionary mapping branch names to leaf structures. .. warning:: ``list_structures`` is deprecated and will be removed in release 5.0.0. Parameters filename : str Path to ROOT file. treename : str, optional (default=None) Name of tree in the ROOT file (optional if the ROOT file has only one tree). Returns ------- structures : OrderedDict An ordered dictionary mapping branch names to leaf structures. """

    warnings.warn("list_structures is deprecated and will be "
                  "removed in 5.0.0.", DeprecationWarning)
    return _librootnumpy.list_structures(filename, treename)