Functions for tree traversal.
Changes in comments.
More general depth-first traversal.
Avoided const troubles by simply not passing a reference

to the graph as argument to the function objects.
Correction.
Factoring out the stack model part.
Minimal symbol table.
Exceptions for the symbol table.
More lumberjack's terminology.
Distinguish trim and prune.
Mistakenly unqualified identifiers.
Added some default constructors.
Separated cpp from hpp.
Parser for a minimal language.
New expressions of the twice sample.