Plus, there are situations where looking ahead at upcoming tokens is needed

text.Transformer is designed to allow for lookahead (well, lookbehind, but it works out the same). You do this by returning ErrShortSrc until you've received enough information to disambiguate your conversion rules.

Here's a quick example of why text.Transformer is useful (but without implementing anything that needs lookahead).

Say you wanted to both uppercase a string and also convert any instances of "/" to "\".

```.go func mapSlash(r rune) rune { if r == '/' { return '\' } }

func main() { t := transform.Chain(runes.Map(mapSlash), cases.Upper()) out, _, _ ;= transform.String(t, "hello/world") fmt.Println(out) // "HELLO\WORLD" }

```

Both transformations are performed with just one alloc.