not(X1, ... Xn)
Prolog negation: Fails if the goal X1, ... Xn succeeds.
If not succeed then this does not mean that the goal is false but only that it does not follow from the rules and the facts which are in the program.
Actually, not can be defined on Prolog and here is one possible definition for this predicate:
not(X) :- X, !, fail.
With not are defined many other predicates. For example:
X\=Y :- not(X=Y).
standard control predicates