The object-oriented implementation of the link, variance and correlation options makes it straightforward to add extra choices. The code uses the built-in link-proto type and defines prototypes gee-varfun-proto and gee-corr-proto. Any object responding to the same messages as these prototypes can be supplied in place of those provided. The easiest way to create new options (for example, an inverse gaussian variance structure) is to modify one of the existing examples. The code documents the messages that each object needs to respond to.