Easing rotation on button event

1

Hi,

I have a button and a MC. I want to achieve the following:

when I click on the button my MC rotate to 180degrees with an easing sin.
when i click again on this button or and other button my MC go back to the
original position with an easing sin.

this is where I am at so far but i cant get the MC to rotate gradually

MovieClip.prototype.rotation = function() {
truc.onRelease = function() {
if (this.bob == true) {
bob._rotation += (180-this._rotation)/9;
}else{
bob._rotation += (180-bob._rotation)/9;
}
}

};
bob.rotation();

also how to back to initial position using the same button?

cheers