subtract(subtrahend) {
    // Make sure we're dealing with complex number.
    const complexSubtrahend = this.toComplexNumber(subtrahend);

    return new ComplexNumber({
      re: this.re - complexSubtrahend.re,
      im: this.im - complexSubtrahend.im,
    });
  }
@param {ComplexNumber|number} subtrahend
@return {ComplexNumber}