case equivalent), may be typed in any window
under the menubutton's toplevel to post the menubutton.
<P><LI value="8">
The F10 key may be typed in any window to post the first menubutton
under its toplevel window that is not disabled.
<P><LI value="9">
If a menubutton has the input focus, the space and return keys
post the menubutton.
<P></OL>
<P>
If the menubutton's state is <B>disabled</B> then none of the above
actions occur:  the menubutton is completely non-responsive.
<P>
The behavior of menubuttons c