I need an advice, please

["Marco Sangiorgi" <sangiorgim@xxxxxxx>]

Hello,

I have a problem in my simulation. I have an EQUATION. In this equation I
calculate a certain value, say X. I calculate a new X at each step of the
simulation. The problem is that I should keep the old value of X in the same
equation in order to perform another calculation.

In other words, in the same EQUATION, at the same time t, I should
calculate:

X = X(t)
diff = X(t) - X(t-1)


I can't suceed in keeping in memory the value of X at the previous time
step, that is X(t-1)

Any suggestion?

Thank you,

Marco