X,Y specifies a conjunction of goals. X,Y succeeds if X succeeds and Y succeeds. If X succeeds and Y fails then an attempt is made to re-satisfy X. If X fails the entire conjunction fails.
Examples
?- true, true.
yes
?- true, fail.
no
?- fail, true.
no
?- fail, fail.
no
?- true, true, true.
yes
?- true, fail, fail.
no
?- fail, true, fail.
no
?- fail, fail, true.
no
?- true, true, fail.
no
?- true, fail, true.
no
?- fail, true, true.
no
?- fail, fail, fail.
no b :- true.
c :- true.
d :- true.
y :- true.
a :- b,c,d.
x :- y,z.
?- a.
yes
?- x.
no p2(1) :- true.
p2(2) :- true.
p2(3) :- true.
p3(a) :- true.
p3(b) :- true.
p3(c) :- true.
p4(1, b, [a,b,c]) :- true.
p4(3, c, [1,2,3]) :- true.
p4(X, Y, [q,w,e,r,t,y]) :- true.
p1(X, Y, Z) :- p2(X), p3(Y), p4(X,Y,Z).
?- p1(X, Y, Z).
X = 1
Y = a
Z = [q,w,e,r,t,y]
yes;
X = 1
Y = b
Z = [a,b,c]
yes;
X = 1
Y = b
Z = [q,w,e,r,t,y]
yes;
X = 1
Y = c
Z = [q,w,e,r,t,y]
yes;
X = 2
Y = a
Z = [q,w,e,r,t,y]
yes;
X = 2
Y = b
Z = [q,w,e,r,t,y]
yes;
X = 2
Y = c
Z = [q,w,e,r,t,y]
yes;
X = 3
Y = a
Z = [q,w,e,r,t,y]
yes;
X = 3
Y = b
Z = [q,w,e,r,t,y]
yes;
X = 3
Y = c
Z = [1,2,3]
yes;
X = 3
Y = c
Z = [q,w,e,r,t,y]
yes
?- p2(X), p2(X), p2(X).
X = 1
yes;
X = 2
yes;
X = 3
yes
?- p2(X), p3(X), p2(X).
no