### Symbolic accumulation

With Maple I can write

```
seq(add(a[i], i=0..n), n=0..3);
```

and get

```
a[0], a[0]+a[1], a[0]+a[1]+a[2], a[0]+a[1]+a[2]+a[3] (*)
```

If I write in Sage

```
for n in range(4):
add(a[i] for i in range(n))
```

I get the NameError: global name 'a' is not defined. If I first introduce the
variable by a = var('a') then I get the TypeError: 'sage.symbolic.expression.Expression' object does not support indexing.

How can I get the line (*) with Sage?