oDlg:DefControl( <oControl> ) -> nil

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 PARAMETER:

  <oControl>   Is a reference to the control's object that will be
               connected with the dialog box


 DESCRIPTION:

  This method is used when a control is added to a dialog box that gets
  it's structural information from a resource. 
  Before the control is added a check is made to see whether or not the 
  ::nID instance variable of <oControl> is unique for this 
  dialog box. If the identifier is already in use a warning will be shown 
  and the control will NOT be added to the ::aControls array.