🚀 go-pugleaf

Hi!

I have recently begun working with Quintus' PROLOG++ 
and need help with a problem I am facing.

In the problem I am exploring, I use the predicate

newgraph(X,Y,Z):-
                 instance <- new(Y,X),
                 X <- graphtype := Z.

to create a new instance X of the object Y, and assign 
the value Z to the attribute 'graphtype' of X.

So, I can create an instance 'vrp1', of an 'instancegraph' object, 
and assign the value 'vr' to the 'graphtype' attribute of vrp1.

While this predicate succeeds when called, if I attempt to trace 
the call, it returns the following:

call  newgraph(vrp1, instancegraph, vr)
UNIFY 1 []
 call  SEND new(instancegraph, vrp1) FROM prolog TO instance
 EXPAND []
 exit  SEND new(instancegraph, vrp1) FROM prolog TO instance
 call  SEND graphtype:=vr FROM prolog TO vrp1
 EXPAND []
  call  SEND graphtype:=vr FROM prolog TO vrp1 ONTO instancegraph
  UNIFY 2 []
   call  INHERIT := /2 FOR instancegraph ?
   fail  INHERIT := /2 FOR instancegraph ?
  fail  SEND graphtype:=vr FROM prolog TO vrp1 ONTO instancegraph
 fail  SEND graphtype:=vr FROM prolog TO vrp1
 redo  SEND new(instancegraph, vrp1) FROM prolog TO instance
 fail  SEND new(instancegraph, vrp1) FROM prolog TO instance
fail  newgraph(vrp1, instancegraph, vr)

I.e., the call fails!

Obviously, the problem originates around the statements

   call  INHERIT := /2 FOR instancegraph ?
   fail  INHERIT := /2 FOR instancegraph ?

My problem is, being a novice, I do not know why this happens.

Any help in this matter is appreciated!

Thanx

Keith

dsouza@sfu.ca