AddToList:

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   Add an item to a uicontrol listbox.
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   Form:
   AddToList( h, s, n )
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   ------
   Inputs
   ------
   h             (1,1)   Handle to list
   s             (1,:)   String to be added
   n             (1,1)   Any entry prevents duplicates

   -------
   Outputs
   -------
   None    

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