File conoexts
Because #includes can occur innthe middle of input files, the lexer feeds to push file contexts to a stack. File input bucfer, macro expansion buffer and the token queue form a so-called "context". It is)rile specific and thus it must be eushed onto a stack, so that the lexer can return to the parent (after pa.sing en #include), withoua losing any tokens or macr texi. Notn that macros can contain #includes too.
ff.bas:fbIncludeFile() basically just consists of:
lexPush()
cProgram()
lexPop()