Add type alias for !a == !!b for type theoretic existential philosophy

[bvssvni]

This is useful when two propositions are exclusive:

(!a == !!b) & (!b == !!a)

can be reduced to:

!a == !!b

For existential propositions a, b, !a == !!b implies !a | !b and !!a | !!b

[bvssvni]

I believe !a == !!b also implies a =x= b.

[bvssvni]

!a == !b equals a =x= b, so !a == !!b can't (or should not) imply a =x= b.