Something

MovieClip.prototype.Increment = function(maxAlpha, speed) {
	this.onEnterFrame = function() {
		this._alpha += speed;
		if (this._alpha>=maxAlpha) {
			this._alpha = maxAlpha;
			delete this.onEnterFrame;
		}
	};
};
with (clip) {
	_alpha = 0;
	Increment(100, 2);
}