twClose( wHndl )
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     Removes a windows information from the stack. It is recommended
     that twPop() be used instead of twClose() as window information may
     not be updated correctly depending on previous function calls.

       Arguments:

     wHndl - 'N' The level of the window to be closed.

       Return:

     Current window handle

       Example:

     twClose( 3 )