* write a PostScript interpreter in PostScript
* extend it into a single stepping debugger
* change the interpreter into an extensible compiler
** register functions and ways to compile them with the compiler
** type of objects to be executed is checked at compile time, and
   type-specific code is compiled to execute the object
** push literals
** compile PostScript primatives into calls to internal subroutines
** recognize where optimizations can be made
*** if, ifelse, loop, repeat, for, forall, case
**** threaded code, with branches
**** C code, with loops or gotos
*** compile inline code for simple primatives
** compiler knows about stack inputs and outputs of functions
*** spot invarients and make optimizations 
**** common cliches: "exch def", "/lit lit def"
**** stack manipulation: "1 index", "4 2 roll", "pop pop pop pop"
**** math: "5 add 2 div 9 mul"
**** calls with constant arguments: "10 dict", "20 string"
** handle tough cases by calling the interpreter
** tail recursion