compound(X) succeeds if X currently stands for a compound term.
Examples
?- compound(a(b,c)).
yes
?- compound(1+1).
yes
?- compound([a,b,c]).
yes
?- compound([]).
no
?- compound(abc).
no
?- compound(1).
no
?- compound(1.5).
no
?- compound(X).
no
?- compound(_).
no