sameSubset(element, ...rest) {
    this._introduce(element);
    const root = this.root(element);
    return rest.every(element => {
      this._introduce(element);
      return this.root(element) === root;
    });
  }
Check if the elements belong to the same subset.
Complexity: O(A^-1) (inverse Ackermann function) amortized.

@param {...*} element
@return {boolean}