WNUM()
 Determines the highest window handle
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 Syntax

     WNUM() --> nHighestWindow

 Returns

     WNUM() determines the highest open window number (handle) so that you
     can iterate a loop over the range of open windows (see Winstore.prg in
     the samples directory).  Remember that any number of handles, from 0 to
     the highest handle number, can be inactive.

 Example

     Open three windows, and then close the second window.  WNUM() returns
     the highest window number, but there are two inactive handles:

     nW1  :=  WOPEN(3, 3, 10, 12)
     nW2  :=  WOPEN(10, 10, 20, 70)
     nW3  :=  WOPEN(5, 5, 22, 60)
     WSELECT(nW2)
     WCLOSE()
     ? WNUM()            // Result corresponds to nW3