function solve() {
  let state = ['Water', 'Fire', 'Earth', 'Air'];

  class Melon {
    constructor(weight, melonSort) {
      if (new.target === Melon) {
        throw new Error(`Abstract class cannot be instantiated directly`);
      }
      this.weight = weight;
      this.melonSort = melonSort;
      this._elemIndex = this.weight * this.melonSort.length;
    }
    get elementIndex() {
      return this._elemIndex;
    }
    toString() {
      let a;
      if (this instanceof Melolemonmelon) {
        a = state[0];
      } else {
        a = this.constructor.name.split('melon')[0];
      }

      return `Element: ${a}\nSort: ${this.melonSort}\nElement Index: ${this.elementIndex}`;
    }
  }
  class Watermelon extends Melon {
    constructor(weight, melonSort) {
      super(weight, melonSort);
    }
  }
  class Airmelon extends Melon {
    constructor(weight, melonSort) {
      super(weight, melonSort);
    }
  }
  class Firemelon extends Melon {
    constructor(weight, melonSort) {
      super(weight, melonSort);
    }
  }
  class Earthmelon extends Melon {
    constructor(weight, melonSort) {
      super(weight, melonSort);
    }
  }

  class Melolemonmelon extends Watermelon {
    constructor(weight, melonSort) {
      super(weight, melonSort);
    }
    morph() {
      let changer = state.shift();
      state.push(changer);
    }
  }
  return { Melon, Watermelon, Firemelon, Earthmelon, Airmelon, Melolemonmelon };
}