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>.

   * This is a non-standard media type, but is commonly used in serving hosted binary files as it is
   * <a href="http://code.google.com/p/browsersec/wiki/Part2#Survey_of_content_sniffing_behaviors">
   * known not to trigger content sniffing in current browsers</a>. It <i>should not</i> be used in
   * other situations as it is not specified by any RFC and does not appear in the <a