X-Git-Url: http://nitlanguage.org

diff --git a/lib/ai/backtrack.nit b/lib/ai/backtrack.nit
index 550df10..597164d 100644
--- a/lib/ai/backtrack.nit
+++ b/lib/ai/backtrack.nit
@@ -40,7 +40,7 @@ module backtrack
 # # Basic search
 #
 # The method `solve` returns a new solver for a backtrack search.
-class BacktrackProblem[S: Object,A]
+abstract class BacktrackProblem[S: Object,A]
 	# The starting state of the problem.
 	# It is this object that will be modified by `apply_action` and `backtrack`.
 	fun initial_state: S is abstract
@@ -209,6 +209,7 @@ class BacktrackSolver[S: Object, A]
 
 				problem.backtrack(state, a)
 				node = node.parent
+				assert node != null
 				continue
 			end