Base nexting on interface satisfaction

This is done unsubtlely at the moment and there is plenty of room for
optimisation of assertion location to prevent repeated reasserting as is
done now.

graph/iterator/value_comparison_iterator.go

@@ -46,7 +46,6 @@ const (
 )
 
 type Comparison struct {
-	Base
 	uid    uint64
 	tags   graph.Tagger
 	subIt  graph.Iterator
@@ -132,7 +131,7 @@ func (it *Comparison) Next() (graph.Value, bool) {
 	var val graph.Value
 	var ok bool
 	for {
-		val, ok = it.subIt.Next()
+		val, ok = graph.Next(it.subIt)
 		if !ok {
 			return nil, false
 		}