question (v5.0)

Can anyone explain this?

working code:

start :-
X = [2,3,4],
Y = [3,2,1,5,6,4],
psubset(X,Y).

psubset([],_).
psubset([H|T],List) :-
   member(H,List),
   psubset(T,List).

At first glance, psubset(X,Y) seems to be equivalent to:
intersection(X,Y,X) and union (X,Y,Y),
(we test each element of X to see if it is a member of Y).

However psubset succeeds in cases 
where intersection and union fail -- see code below.
Do they fail because they can't handle 
structures in the lists Children and Constraints, or
is it due to other reasons?  Can intersection or union
be massaged into working for this code?

-- doug edmunds
24 June 2000


% code starts

:- use_module(library(fd)).
:- use_module(library(propia)).

:- writeln("type this: start(Children,most)").

start (Children,Language) :-

Children = 
[
child(2,_,_),
child(3,_,_),
child(4,_,_),
child(5,_,_),
child(6,_,_)
],

[Keith,Nora,Margo, Otto, Ivey,Fell,Hall]::[2..6],

Constraints =
[
child(_ , libby, jule),
child(Keith , keith, _ ),
child(Ivey, _, ivey),
child(Ivey, _, ivey),
child(Nora, nora, _ ),
child(Fell, _, fell),
child(Margo, margo, _ ),
child(Otto, otto, _ ),
child(Hall, _, hall),
child(_, _, gant)

],

one_older(Keith, Ivey) infers Language,
one_older(Ivey, Nora) infers Language,
three_older(Fell, Margo) infers Language,
twice_older(Otto, Hall) infers Language,

psubset(Constraints, Children).

% union(Constraints, Children, Children). %produces 'No'
% intersection(Constraints, Children, Constraints). % produces 'No'

one_older (Old, Young) :- Old #= Young + 1.
three_older(Old, Young) :- Old #= Young + 3.
twice_older(Old, Young) :- Old #= Young * 2. 

psubset([],_).
psubset([H|T],List) :-
member(H,List),
psubset(T,List).

%code ends