arg0( +Index, +Term , -Arg )*
Similar to arg//33, but arg0(0,_T_,_F_) unifies F with T's principal functor:
arg0(0,_T_,_F_)
?- arg0(0, f(a,b), A). A = f. ?- arg0(1, f(a,b), A). A = a. ?- arg0(2, f(a,b), A). A = b.
1. prolog::arg0_3::arg0/3(int ARG1, int ARG2, int ARG3)():