close (OpenGoal (Simple (Ext     _) _) _ ) =
  error "Distribution.Solver.Modular.Dependency.close: called on Ext goal"
close (OpenGoal (Simple (Ext     _) _) _ ) =
  error "Distribution.Solver.Modular.Dependency.close: called on Ext goal"
close (OpenGoal (Simple (Ext     _) _) _ ) =
  error "Distribution.Solver.Modular.Dependency.close: called on Ext goal"