have \
                             the same identifier. \
                             The collection is never empty.

        resolvelib >=0.6.0

        :param identifier: The value returned by ``identify()``.

        :param requirements: The requirements all returned candidates must satisfy.
            Mapping of identifier, iterator of requirement pairs.

        :param incompatibilities: Incompatible versions that must be excluded
            from the returned list.

        :returns: An iterable that orders candidates by preference, \
                  e.g. the most preferred candidate comes first.
        r7