BackwardsSearch does not support constrained problems

[simonblauth]

The construction of BackwardSearchGoal fails when goal is a StateConstrainedGoal. Here is a minimal (not) working example:

using PDDL
import SymbolicPlanners as SP

domain = @pddl("(define (domain d)
                (:predicates
                    (something)
                )
                (:action do-something
                    :parameters ()
                    :precondition (and (not (something)))
                    :effect (and something)
                )
                )")
problem = @pddl("(define (problem p) (:domain d)
                 (:init (not (something)))
                 (:goal (and (something)))
                 (:constraints (and))
                 )")

planner = SP.BackwardPlanner(heuristic=SP.HAddR())
planner(domain, initstate(domain, problem), PDDL.get_goal(problem)) # works
planner(domain, initstate(domain, problem), SP.StateConstrainedGoal(problem)) # errors