'Establish instance variables with default values'

def __init__(self):

    self._rels = (('rId1', 'http://reltype1', 'docProps/core.xml', 'Internal'), ('rId2', 'http://linktype', 'http://some/link', 'External'), ('rId3', 'http://reltype2', '../slides/slide1.xml', 'Internal'))
