Equivalent to “there exists an X such that P is
true”, thus X is normally an unbound variable. The use of the
explicit existential quantifier outside
Equivalent to simply calling P.
Depends on P.
Call errors (see ref-sem-exc).
bagof/3 without and with the existential quantifier:
| ?- bagof(X, foo(X,Y), L). X = _3342, Y = 2, L = [1,1] ; X = _3342, Y = 3, L =  ; no | ?- bagof(X, Y^foo(X,Y), L). X = _3342, Y = _3361, L = [1,1,2] ; no