Merge pull request #19 from dmb2168/master

Bringing chris_kira_SA up to date with master
added string literal to tree conversion. verification needed
fixes to internal 0degr trees; lrxlib null children fix. added 2nd test failing
added print nulls back in
fixed ordering of children within tree
updated tests to reflect correct ordering
Merge pull request #20 from dmb2168/chris_kira_semantic_analysis

Fixes to Tree Assignment And Lrx