anchor.diff:        case ')':
           floor--;
           break;
  +      case '\n':
  +        break;
         default:
           throw 'Invalid instruction at index $i: \'${instructions[i]}\'.';
       }
  -    if (floor == -1) return i + 1;
  +    i++;
     }
  -  return null;
  +
  +  return floor == -1 ? i : null;
   }
  
positive.code:       case ')':
          floor--;
          break;
        default:
          throw 'Invalid instruction at index $i: \'${instructions[i]}\'.';
      }
    }
    return floor;
  }
  
  
negative.code: /// Throws an [Exception] if an invalid instruction is found.
  int partTwo(String instructions) {
    var floor = 0;
  
    var i = 0;
    while (i < instructions.length && floor != -1) {
      switch (instructions[i]) {
        case '(':
          floor++;
  