Fix null pointer error in type checker when tuple type contains null type
Fixed bug in lifting tuples
Turing machine busy beaver example
Fix unguarded pre check to treat all node calls as unguarded
Avoid unguarded pre expressions
Fixed unguarded pres in example
Add enumeration types. Require constants and enumeration values to be globally unique.
Remove unused variable
Merge branch 'master' (early part) into junit
Merge with master
Merge remote-tracking branch 'origin/junit' into junit