/* Domain theory  for explanation-based learning (ebg.pl) */

on(o1,o2).

isa(o1,box).
isa(o2,table).
isa(o3,box).

colour(o1,red).
colour(o2,blue).

volume(o1,1).
volume(o3,6).

density(o1,2).
density(o3,2).

:-dynamic safe_to_stack/2.  % needed to allow dynamic modifications

safe_to_stack(X,Y) :- lighter(X,Y).

lighter(O1,O2) :- weight(O1,W1), weight(O2,W2), W1 < W2.

weight(O,5) :- isa(O,table).
weight(O,W) :- volume(O,V), density(O,D), W is V * D.


/* Examples to prove and explain: */

/*
safe_to_stack(o1,o2).
safe_to_stack(o2,o3).
safe_to_stack(o1,o3).
*/