snippet for "why can't I subtitude functor with variable in a predicate" @ stackoverflow

students.pl

% facts : sname(ID,NAME)
sname(1,ada).
sname(2,bob).

% facts : point(ID,POINT)
point(1,80).
point(2,81).

% working search in example
search_by_name(Key,Value) :-
	Key == name,
	sname(ID,Value),
	point(ID,Point),
	write(Value),write(Point),nl.

% a more general search I want to write
% but not accepted by BProlog
% search_by_attr(Key,Value) :-
% 	Key(ID,Value),
% 	sname(ID,Name),
% 	point(ID,Point),
% 	write(Name),write(Point),nl.

% a working one
search_by_attr(Key,Value) :-
	call(Key,ID,Value),
	sname(ID,Name),
	point(ID,Point),
	write(Name),write(Point),nl.