Add new fields to the current reference.

def _add_to_ref(self, rec_curr, line):
        """Add new fields to the current reference."""
        # Examples of record lines containing ':' include:
        #   id: GO:0000002
        #   name: mitochondrial genome maintenance
        #   namespace: biological_process
        #   def: "The maintenance of ...
        #   is_a: GO:0007005 ! mitochondrion organization
        if line[:4] == "id: ":
            assert not rec_curr.item_id
            item_id = line[4:]
            rec_curr.item_id = item_id
            rec_curr.id = item_id
        elif line[:8] == "alt_id: ":
            rec_curr.alt_ids.add(line[8:])
        elif line[:6] == "name: ":
            assert not rec_curr.name
            rec_curr.name = line[6:]
        elif line[:11] == "namespace: ":
            assert not rec_curr.namespace
            rec_curr.namespace = line[11:]
        elif line[:6] == "is_a: ":
            rec_curr._parents.add(line[6:].split()[0])
        elif line[:13] == "is_obsolete: " and line[13:] == "true":
            rec_curr.is_obsolete = True
        elif self.optobj and ':' in line:
            self.optobj.update_rec(rec_curr, line)