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/optional_iterator.go

@@ -30,8 +30,6 @@ import (
 	"fmt"
 	"strings"
 
-	"github.com/barakmich/glog"
-
 	"github.com/google/cayley/graph"
 )
 
@@ -78,16 +76,6 @@ func (it *Optional) Clone() graph.Iterator {
 	return out
 }
 
-// FIXME(kortschak) When we create a Nexter interface the
-// following two methods need to go away.
-
-// Nexting the iterator is unsupported -- error and return an empty set.
-// (As above, a reasonable alternative would be to Next() an all iterator)
-func (it *Optional) Next() (graph.Value, bool) {
-	glog.Errorln("Nexting an un-nextable iterator")
-	return nil, false
-}
-
 // DEPRECATED
 func (it *Optional) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())