_twHandle( wHndl )
 ----------------------------------------------------------------------------

     Get/Set the window handle for addressing of library
              functions.

       Arguments:

     wHndl  - 'N' Window handle to set. Defaults to the current handle.

       Return:

      Current window handle

       Example:

      win1 := twOpen()
      win2 := twOpen( 5, 5, 10, 10, "w+/b")

      // write to win2
      twSay( 2, 2, "I'm window number 2" )
      // write to win1
      _wHandle( win1 )
      twSay( 2, 2, "I'm window number 1" )