anchor.diff:      if (specializeFieldAccess(node)) return;
       if (specializeIndexableAccess(node)) return;
       if (specializeArrayAccess(node)) return;
  -    if (specializeSingleUseClosureCall(node)) return;
       if (specializeClosureCall(node)) return;
   
       AbstractValue receiver = getValue(node.receiver.definition);
  
positive.code: import '../world.dart' show World;
  import 'cps_fragment.dart';
  import 'cps_ir_nodes.dart';
  import 'cps_ir_nodes_sexpr.dart' show SExpressionStringifier;
  
  enum AbstractBool {
    True, False, Maybe, Nothing
  
negative.code:     values.remove(node.variable);
    }
  }
  