Unification first compares the top-level structure of the LHS and RHS
types.
Both are arrays of the same length; they unify if the element types unify.
Both element types are structs; they unify if they have
the same number of fields with the same names and if the
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