code: function multiply(that) {
        if (this.isIdentity()) {
          return that;
        }
  
        if (that.isIdentity()) {
          return this;
        }
  
        return new this.constructor(this.a * that.a + this.c * that.b, this.b * that.a + this.d * that.b, this.a * that.c + this.c * that.d, this.b * that.c + this.d * that.d, this.a * that.e + this.c * that.f + this.e, this.b * that.e + this.d * that.f + this.f);
      }
docs: multiply
  
  @param {module:kld-affine.Matrix2D} that
  @returns {module:kld-affine.Matrix2D}