Add unary_! to BooleanFormula
Add conjoin and disjoin of variable params
Restore Heirarchy Code
Move object SetBoolenaFormula before the class
This seems to be more standard
Add invariant to SetBooleanFormula
The underlying set is not allowed to be null
Add intesect method to class SetBooleanFormula
Not sure why I need to but I do
Improve disjoin and conjoin
Short circuit, and filter