lp.txt

- current:
envFrame ; environment stack (AND; nested predicate calls)
choice-point frame ; choice point stack (OR; backtracking search)

	frame-object <- frame-object ..

trail ; variable binding (to be unwound on backtrack)

 
FrameObject
	< EnvironmentFrame
	< GenericChoiceFrame; represent OR state of search
		- ctu; program, arg cntr		

		< ChoicePointFrame; OR part of search tree
		- argVars

		< GenericTripleMatchFrame; represent state of some triple match
			< TripleMatchFrame; represent state of concrete triple match
			< ConsumerChoicePointFrame; represent results of a tabled predicate

bind: add binding to trail
	var.value = var (isRef)
unwind:
	unbind var
	remove from trail
deref:
	find original var



{ ?a p1 ?b } => { ?a p2 ?b }

{ ?k p2 ?l } => { ?k p3 ?l }

{ ?x p3 ?y }?