<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_ref(self, reftype, data):
 """Add a reference and returns the identifier."""

        ref = (reftype, data)
        try:
            index = self.refs.index(ref)
        except ValueError:
            self.refs.append(ref)
            index = len(self.refs) - 1
        return str(index)