reset(a, b, life, easing) {
    this.same = b === null || b === undefined;
    this.a = Span.setSpanValue(Util.initValue(a, 1));
    this.b = Span.setSpanValue(b);

    life && super.reset(life, easing);
  }
Resets this behaviour's parameters.
@param {number|Span} [a=1] - The initial alpha value or range.
@param {number|Span} [b] - The final alpha value or range. If not provided, it will be the same as 'a'.
@param {number} [life] - This behaviour's life.
@param {string} [easing] - This behaviour's easing function.