Hum... this only takes into account the "type" falg, but discards everything else (say, padding and whatnot). I think: something along the lines of:
```D
FormatSpec!char fmt2 = fmt;
put(sink, "[");
formatValue(sink, val.a, fmt);
put(sink, "..");
formatValue(sink, val.b, fmt2);
put(sink, ")");
```

This is more generic and robust (I think): In particular, it handles explicit width for those with OCD that want alignment. Unfortunately, it doesn't work so well for handling the display of the base :/