Used internally to build a list for mapping between a random number and 
        a song index.

def _gen_shuffles(self):
        '''
        Used internally to build a list for mapping between a random number and 
        a song index.
        '''
        # The current metasong index 
        si = 0
        # The shuffle mapper list
        self.shuffles = []
        # Go through all our songs...
        for song in self.loop:
            # And add them to the list as many times as they say to.
            for i in range(song[1]):
                self.shuffles.append(si)
            si += 1