ned, and for a "named" requirement, the index(es) should be
        consulted to find concrete candidates for this requirement.

        The return value should produce candidates ordered by preference; the
        most preferred candidate should come first. The return type may be one
        of the following:

        * A callable that returns an iterator that yields candidates.
        * An collection of candidates.
        * An iterable of candidates. This will be consumed immediately into a
          list of candidates.
        r