[AMPL 25057] error with the mod command: I is already defined

Hello together,
so basically I got the variable var I and also s.t. I which has to be like that based on the task we got from our prof.

But if I put in the command for my mod, I get following error:

I is already defined

context: s.t. >>> I[ <<< t-1]+q[t]-d[t]=I[t];

For further understanding, I will send you my whole mod:

set T:= 1…4; #Anzahl der Perioden

param d; #Nachfrage in Periode

param I0; #Anfangslagerbestand

param s; #Rüstkostensatz

param h; #Lagerhaltungskostensatz

param M; #Eine hinreichend große Zahl

var q {t in T}>=0;

var x {t in T} binary;

var I {t in T}>=0;

minimize Kosten: sum{t in T} (sx[t]+hI[t]);

s.t. I[t-1]+q[t]-d[t]=I[t];

s.t. q[t]<=M*x[t];

s.t. q[t]>=0;

s.t. I[t]>=0;

s.t. x[t] binary;

Also another question: How come that some commands are in purple and some not ?

Thanks in Advance.

Anwar

Answered at https://discuss.ampl.com

Hello to you all,

I cannot access your reply.

Thank you.