Are There Any Examples of User Defined Functions Other Than the Ones in the Advanced Programming Guide?
Virtually all of the system defined functions that come ready to use with CLIPS utilize the same programming interface that you have to use to define your own functions. Look through files in the CLIPS source code such as bmathfun.c, iofun.c, miscfun.c, prdctfun.c, emathfun.c, filecom.c, and multifun.c for examples.