Organize example code.
Add new example code.
Start HW2.
Add writeup file.
Add one more example.
Shaky start. Lots of things are wrong here.
Finish exercise 1.
Reorganize a few lines.
Start on second exercise.
Add new types to represent macros and the language state.
Add part of semantic function that is same as that in StackLang.
Add another stack to play with.
Fix semantic function to use State, not Stack.
Minor change.
Implement DEF and CALL, complete with error checking.