private static <K, V, MB extends ImmutableMultimap.Builder<K, V>> MB combine(MB a, MB b) {
        a.putAll(b.build());
        return a;
    }
Returns a {@code Collector} that accumulates {@code Map.Entry<K, V>} input elements into a
{@code Multimap<K, V>} of the supplied type.

@param <K>             the type of map keys
@param <V>             the type of map values
@param <M>             a supplier of your multimap type
@param multimapFactory a {@code Supplier} which return a new, empty {@code Multimap} of the appropriate type.
@return a {@code Collector} which collects map entry elements into a {@code Multiamp}, in encounter order.
guava/src/main/java/org/jdbi/v3/guava/GuavaCollectors.java