Define another complicated function

real x1=0.0;
real I1=I(x1,t)-0.5;
where I is the solution of an system
My question is: in FF++ how we define the function I1? Please.
The difficulties is that FF++ don’t understant I(x1,t)

Thank you for the help

Can any one help me please
i try

func real I1(real x1,real t){return (I(x1,t)-0.5);}

but it’s not good