How can I decode a single character from a vector of octets in common lisp? 

I want something like:

```
(decode-character vector :start i :encoding :utf-8)

```

or more specifically:

```
(decode-character #(195 164 195 173 99 195 176) :start 0)
=> #\LATIN_SMALL_LETTER_A_WITH_DIAERESIS

```

which would return the UTF-8 encoded character that starts at position `i` in `vector`. 

I can't figure out how to do that using either babel or flexi-streams.