anchor.diff:      while (configuration != null) {
         configuration = configuration.step(this);
       }
  +    ;
     }
   
     Configuration exec(Statement statement, ExecConfiguration conf) =>
  
positive.code:     return parent.lookupBinding(variable);
    }
  
    Value lookup(VariableDeclaration variable) {
      return lookupBinding(variable).location.value;
    }
  
negative.code:     assert(!contains(variable));
      return new Environment(this)
        ..bindings.add(new Binding(variable, new Location(value)));
    }
  
    Environment extendWithThis(ObjectValue v) {
      assert(!containsThis());
      return extend(new VariableDeclaration('this'), v);
    }
  }
  
  