anchor.diff:    }
   
     Value visitInvalidExpression1(InvalidExpression node, env) {
  -    throw new InvalidExpressionError(node);
  +    throw 'Invalid expression at ${node.location.toString()}';
     }
   
     Value visitVariableGet(VariableGet node, env) {
  
positive.code:   }
  }
  
  class Evaluator extends ExpressionVisitor1<Value> {
    Value eval(Expression expr, Environment env) => expr.accept1(this, env);
  
  
negative.code: class Interpreter {
    Program program;
    StatementExecuter visitor = new StatementExecuter();
  
    Interpreter(this.program);
  
  