Who use the variable 'OLD(indvar)'
 

In my problem, B.C. is that Robin B.C. which is mixing Dilichlet and Neumann B.C.

So, I discretized that equation

Cn=Cs-2dr*f(z)*Cp

I want to code that equation

Cs=SOUTH(C1)

Cp=OLD(C2)

but When I use the variable OLD(C1), PHOENICS can't calculate.

Is the OLD can't use in Boundary?

Thank you for your concentration.