public FastSparseSet<Integer> get(int key) {

    int index = 0;
    if (key < 0) {
      index = 2;
      key = -key;
    }
    else if (key >= VarExprent.STACK_BASE) {
      index = 1;
      key -= VarExprent.STACK_BASE;
    }

    FastSparseSet<Integer>[] arr = elements[index];

    if (key < arr.length) {
      return arr[key];
    }
    return null;
  }
public FastSparseSet<Integer> get(int key)