Of course. The notation:
Constraint: sub (dt 1 x1) (dt 0 u) (dt 0 fx)
Means there is a subtraction reaction where x1' = u - fx.
The 'dt' and the following number shows the derivative of the following variable. In my system, if a variable has a derivative of 1, it is a species. Hence,
Constraint: sub (dt 0 fx) (dt 0 f12) (dt 0 f21)
does not contain any species. The formula is fx = f12 - f21.
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