TreeGrammarDecomposition: fixed a bug which could lead to some issues for formulas having variables with indexes != non-terminals
did a clean-up of unnecessary comments
defined for the whole class 2 specific non-terminal-strings

git-svn-id: http://gapt.googlecode.com/svn/trunk/source@2109 85d21ef8-add0-11de-ae77-5bfb9c5a3f34