Closure control format - %cc
Sometimes, especially when an edit box is displayed, it is desirable to control
the closure of a window. %cc takes a call-back function as its argument. This
function is called before the window closes (for whatever reason). If it
returns a positive value the window is not closed. If a grave is used (%`cc)
then the associated call-back function is called after the window is closed. A
single window could use both %cc and %`cc.