How can I formulate this conditional constraint for cplex :

```
if (S[i][t] <=0 ) then   S[i][t+1]=S[i][t]-livraison[i][t]+order[i][t]

```

with `S[i][t]` and `order[i][t]` are a decision variables and  

`livraison[i][t]` is a known parameter