unifying <code>[]P</code> and <code>[]string</code> and
thus <code>P</code> and <code>string</code>.
Since the type argument for <code>P</code> is known at this point
(there is a map entry for <code>P</code>), its type argument
<code>string</code> takes the place of <code>P</code>.
And since <code>string</code> is identical to <code>string</code>,
this unification step succeeds as well.