codegen functions for elements of the TM program

  genRunTimeSystem()
  genPrintFunction()
  genFunction()
    - for Module 5, perhaps only genMain()

utility functions to use in other codegen functions
  genRO(...)
  genRM(...)
  genComment(...)

codegen functions for function definitions and calls

  genFunction()
    genCallingProc()
    genBody()
    genReturnProc()

  genFunctionCall()      -- a kind of expression
    genCallingProc()
    genJump()
    genReturnProc()