Added Gremlin tests for the Except operator.

graph/iterator/not_iterator.go

@@ -146,11 +146,11 @@ func (it *Not) Type() graph.Type { return graph.Not }
 
 func (it *Not) Optimize() (graph.Iterator, bool) {
 	// TODO - consider wrapping the primaryIt with a MaterializeIt
-	if optimizedPrimaryIt, optimized := it.primaryIt.Optimize(); optimized {
+	optimizedPrimaryIt, optimized := it.primaryIt.Optimize()
+	if optimized {
 		it.primaryIt = optimizedPrimaryIt
-		return it, true
 	}
-	return it, false
+	return it, optimized
 }
 
 func (it *Not) Stats() graph.IteratorStats {

query/gremlin/gremlin_test.go

@@ -120,6 +120,20 @@ var testQueries = []struct {
 		tag:    "acd",
 		expect: []string{"D"},
 	},
+	{
+		message: "use Except to filter out a single vertex",
+		query: `
+			g.V("A", "B").Except(g.V("A")).All()
+		`,
+		expect: []string{"B"},
+	},
+	{
+		message: "use chained Except",
+		query: `
+			g.V("A", "B", "C").Except(g.V("B")).Except(g.V("C")).All()
+		`,
+		expect: []string{"A"},
+	},
 
 	// Morphism tests.
 	{