evidence that non-finite clauses have a phonologically null pro subject comes from the fact that predicate nominals must agree with the subject of a copular clause. this is illustrated in (5) and (6). example (5) shows that the number of the predicate nominal must agree with that of the overt subject: in (5a) the singular subject ("their son") requires a singular nominal predicate ("millionaire"); in (5b), the plural subject ("his sons") requires a plural nominal predicate ("millionaires"). the examples in (6) show that the same contrast holds of pro subjects: if pro is controlled by a singular antecedent, in (6a) the subject of "want", then the predicate nominal must be singular; if pro is controlled by a plural antecedent, as in (6b), then the predicate nominal must be plural.