FIX

The FIX and FREE statements are used to constrain (fix) and unconstrain (free) variables. A variable which has been fixed has a known value assigned to it, and ASCEND will not attempt to change that value. On the other hand, with free variables, ASCEND will try to determine a way to calculate the value, using equations defined in the MODEL and the known values of other variables that have already been solved or fixed.

(* add this to the example on the FIX page *)
METHOD solve_x;

    (* reconfigure the model to solve for x such that y=0 *)
    FREE x;
    FIX y;
    y:= 0;

END solve_x;

See also FREE.