The \+ X goal succeeds if an attempt to satisfy the goal represented by the term X fails. The \+ X goal fails if an attempt to satisfy the goal represented by the term X succeeds.
Examples
?- \+ true.
no
?- \+ fail.
yesNote: "not" is a synonym for "\+".
?- not(true).
no
?- not(fail).
yes
?- \+ [A,B,C,9]=[1,2,3,4], A=6, B=7, C=8.
A = 6
B = 7
C = 8
yes
?- \+ ((X=Y,1>2)), X=1, Y=2.
X = 1
Y = 2
yes test1(X,Y) :- \+ ((X=Y,1>2)), X=1, Y=2.
?- test1(X,Y).
X = 1
Y = 2
yes test2(X) :- \+ \+ X=1, X=2.
?- test2(X).
X = 2
yes
?- test2(1).
no
?- test2(2).
no