n alpha mask specify total opaqueness everywhere.
<p>
You can specify <b>-</b> as the value of this option and the alpha
mask will come from Standard Input.  If you do this, don't specify
Standard Input as the source of any other input image.

<DT><B>-invert</B>
<DD>
This option inverts the sense of the values in the alpha mask, which 
effectively switches the roles of the overlay image and the underlying
image in places where the two intersect.

<DT><B>-opacity=</B><i>opacity</i>
<DD>
This option tells how