In reply to Tim.

I don't know. This article is not about link functions nor linear models. As I said, PROC GENMOD supports the FWDLINK and INVLINK functions for custom links. It also supports keywords such as _XBETA_. So, for example, you can define a inverse link function as
INVLINK ilink= _XBETA_;
The programming statements in RPOC GENMOD enable you to use DATA-step syntax to define mathematical expressions.