Add probability logic

[abrudz]

By defining AND as multiplication (since the probability that two event will happen is the product of their probabilities) and NOT as 1-x (since the probability that an event won't happen is 100% minus its probability), all other logic/probability operations follow from De Morgan's laws:

and = x∧y = xy
not = ¬x = 1-x
or = x∨y = ¬((¬x)∧(¬y))
implies = x→y = ¬(x∧¬y)
iff = x↔y = (y→x)∧(x→y)

[csicar]

Further reading: https://en.wikipedia.org/wiki/Boolean_algebra#Basic_operations