1 | initial version |

A simple solution is to use tuples as indices, that is, `x[(i,j,t)]`

instead of `x[i,j,t]`

. They may have as many elements as you like.

2 | No.2 Revision |

A simple solution is to use tuples as indices, that is, `x[(i,j,t)]`

instead of `x[i,j,t]`

. They may have as many elements as you like.

As for the `sum()`

, it is enough to call it once, no matter how many summation indices you have inside - e.g.:

```
sum( x[(i,j,t)] for t in range(10) for i in range(5) for j in range(5) )
```

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