'Return a copy of the identifiers dictionary.
The dict is small, and the penalty for using a reference where a copy is
needed is large. Also, we don\'t want any manipulations of the returned
dict to show up in the book.'

def get_identifiers(self):

    ans = object.__getattribute__(self, '_data')['identifiers']
    if (not ans):
        ans = {}
    return copy.deepcopy(ans)
