anchor.diff:      );
     }
   
  +  /// Updates the state to indicate that the control flow path is unreachable.
     State<Variable, Type> exit() {
       return State<Variable, Type>._(
         false,
  
positive.code:     return this;
    }
  
    State<Variable, Type> promote(
      TypeOperations<Variable, Type> typeOperations,
      Variable variable,
  
negative.code:     return result;
    }
  
    /// Removes a set of [variable]s from a "promoted" [map], treating the map as
    /// immutable.
    Map<Variable, Type> _removePromotedAll(
      Map<Variable, Type> map,
      Set<Variable> variables,
  