Modules of the example:

- LangSem.hs   semantic functions of the original language
- LangDef. hs  definition of the grammar of the original language
- LangExt.hs   extension (grammar + semantics) of the language
- Utils.hs     some definitions shared by LangDef and LangExt
- prog.src     program with the syntax of the original language
- prog2.src    program with the extended syntax
- Main.hs      creates and executes compilers of the original and extended languages
