Caret (^) - call-back function is called when the topmost child window changes.
Grave accent (`) - used to provide access to the handle of the current child
width and height give the width and height of the frame in pixels.
If %fr is preceded by %pv - Pivot then the frame can be re-sized at run time.
ctrl is returned as the handle of the currently selected child window
(see %hw - Handle) or zero if none is selected.
A given window cannot contain more than one frame.
The call-back function is called with the 'reason' MDI_PAGE_TURN (see
clearwin_string@) whenever the topmost child window changes. Typically this
would be used in conjunction with the grave accent to get the handle of the
topmost window in a variable.