multiply(multiplicand) {
    // Make sure we're dealing with complex number.
    const complexMultiplicand = this.toComplexNumber(multiplicand);

    return new ComplexNumber({
      re: this.re * complexMultiplicand.re - this.im * complexMultiplicand.im,
      im: this.re * complexMultiplicand.im + this.im * complexMultiplicand.re,
    });
  }
@param {ComplexNumber|number} multiplicand
@return {ComplexNumber}