/*
   select_first(X,Xs,Ys) :- 
	Ys is the list obtained by removing the 
	first occurrence of X from the list Xs.
*/
     select_first(X,[X|Xs],Xs).
     select_first(X,[Y|Ys],[Y|Zs]) :- X \== Y, select_first(X,Ys,Zs).

%  Program 7.4    Selecting the first occurrence of an element from a list