grompp, a preprocessor for simulation input files for GROMACS (a fast, free, open-source code for some problems in computational chemistry) which calls the system C preprocessor (or other preprocessor as determined by the simulation input file) to parse the topology, using mostly the #define and #include mechanisms to determine the effective topology at grompp run time.
Preprocessor, template engines used in conjunction with a compiler
Slides in HTML format with English narration in RealAudio format are also available to get a jump start learning the architecture and module, preprocessor and embedding developments.