Wiping problem

im wondering how to (whilst mousing down the whole time) move to the next frame whilst moving on and off the image/movieclip/hotspot

An example would be that of wiping condensation from a window, each time you take more off…

i found a this code:

on (dragOver) {
this._alpha = this._alpha + 1;
}

except i need to instead of change the alpha, change the frame to the next each time a user drags over the hotspot

really appreciate if anyone can help me

ta very much