twSetGlobal( nHndl , xInGlobal )
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     twSetGlobal() can be used to get/set the value of a predefined
     TSDWIN global variable. It acts like Clipper's SETCOLOR().

       Arguments:

     nHndl     - 'N' The handle of the global variable which is the
                     index of the array passed with twInitGlobal() or
                     the handle returned by twAddGlobal().

     xInGlobal - 'X' A new value to replace the old one. This parameter
                     is optional.

       Return:

     The old value of the global variable.

       Example:

     nOldValue := twSetGlobal( 3, nNewValue )
     ...
     //do some operations
     ...
     twSetGlobal( 3, nOldValue )