Adifor 3.0 is a source-to-source transformation tool used to augment programs that compute derivatives. As part of the transformation process, Adifor analyzes certain aspects of program behaviour. Furthermore, that analysis depends on the original program being type correct. Since standard Fortran is officially monomorphic, the assumption of type correctness is not normally a difficult constraint to satisfy. There is a classof (non-standard) Fortran programs, however, that take advantage of the pass-by-reference semantics to be, in effect, polymorphic. This report details some techniques for differentiating these polymorphic programs using monomorphic Adifor 3.0. In particular, we report on our efforts to compute derivatives for the structural analysis code STAGS.