---------------------------------------------------------------------------
   Subtract two objects of class acstate
---------------------------------------------------------------------------
   Form:
   c = minus( a, b )
---------------------------------------------------------------------------

   ------
   Inputs
   ------
   a           (:) Object of class acstate
   b           (:) Object of class acstate

   -------
   Outputs
   -------
   c           (:)  Object of class acstate

---------------------------------------------------------------------------