### Problem with Words when the alhabet is not made of "simple" letters

Why do i get:

~~sage : ~~sage: W = FiniteWords(['a','b'])
~~sage : ~~sage: [w for w in W.iterate_by_length(2)]

returns as I wish

```
[word: aa, word: ab, word: ba, word: bb]
```

but

~~sage : ~~sage: W = FiniteWords(['a1','a2'])
~~sage : ~~sage: [w for w in W.iterate_by_length(2)]

returns

```
[word: a1,a1, word: a1,a2, word: a2,a1, word: a2,a2]
```

I do not want the commas to appear here, how can I get

```
[word: a1a1, word: a1a2, word: a2a1, word: a2a2]
```

instead?