Add eval
Add Meaning
Output prompt in evaluator
Add UserDefinedWord evaluation
Add program counter and call stack to Environment
Fix typo
Fix Meaning
Rename lookupWord to lookupXt
A UserDefinedWord has TokenBlock instead of [PToken]
Implement jump
Rename normalProcedure to nativeProcedure
Add nop
Add startCompile and endCompile
Add doctest's $setup section
rename PData to PValue
Implement compiler
Maybe CompileProc will fail
Implement if-then-else
Add error message of jump
Fix the output format of . and .s