'If val is empty, deletes identifier of type typ'

def set_identifier(self, typ, val):

    (typ, val) = self._clean_identifier(typ, val)
    if (not typ):
        return
    identifiers = object.__getattribute__(self, '_data')['identifiers']
    identifiers.pop(typ, None)
    if val:
        identifiers[typ] = val
