This paper describes CREC, a library of C functions for compiling and executing REC programs whose operators and predicates are provided by the caller. Given a C program with a menu-based interactive interface, REC can be added as a programming language to the user interface by assigning menu choices to REC operators and predicates through entries in a table, and linking in the appropriate functions in CREC. Being a very concise language, REC programs are easy to write in an interactive context. The same conciseness allows the compiler to require only a small amount of space. Experience whereby the library benefitted some mathematics and physics programs is described.