field types unify.
The type argument for <code>P</code> is not known yet (there is no map entry),
so unifying <code>P</code> with <code>string</code> adds
the mapping <code>P ➞ string</code> to the map.
Unifying the types of the <code>list</code> field requires
unifying <code>[]P</code> and <code>[]string</code> and
thus <code>P</code> and <code>string</code>.