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/hasa_iterator.go

@@ -46,7 +46,6 @@ import (
 // a primary subiterator, a direction in which the triples for that subiterator point,
 // and a temporary holder for the iterator generated on Check().
 type HasA struct {
-	Base
 	uid       uint64
 	tags      graph.Tagger
 	ts        graph.TripleStore
@@ -159,7 +158,7 @@ func (it *HasA) Check(val graph.Value) bool {
 // another match is made.
 func (it *HasA) GetCheckResult() bool {
 	for {
-		linkVal, ok := it.resultIt.Next()
+		linkVal, ok := graph.Next(it.resultIt)
 		if !ok {
 			break
 		}
@@ -198,7 +197,7 @@ func (it *HasA) Next() (graph.Value, bool) {
 	}
 	it.resultIt = &Null{}
 
-	tID, ok := it.primaryIt.Next()
+	tID, ok := graph.Next(it.primaryIt)
 	if !ok {
 		return graph.NextLogOut(it, 0, false)
 	}