Replace graph map with slice.

graph.go

@@ -1,20 +1,16 @@
 package rdf
 
 type Graph struct {
-	triples map[*Triple]struct{}
+	triples []*Triple
 }
 
 func NewGraph(ts ...*Triple) *Graph {
-	triples := make(map[*Triple]struct{})
-	for _, t := range ts {
-		triples[t] = struct{}{}
-	}
-	return &Graph{triples}
+	return &Graph{triples: ts}
 }
 
 func (g *Graph) Add(s, p, o Node) {
 	t := &Triple{s, p, o}
-	g.triples[t] = struct{}{}
+	g.triples = append(g.triples, t)
 }
 
 // Find returns the first triple matching the given pattern.
@@ -43,13 +39,18 @@ func (g *Graph) FindAll(s, p, o Node) []*Triple {
 }
 
 func (g *Graph) Remove(t *Triple) {
-	delete(g.triples, t)
+	for i, other := range g.triples {
+		if t.Equal(other) {
+			g.triples = append(g.triples[:i], g.triples[i+1:]...)
+			return
+		}
+	}
 }
 
 func (g *Graph) iter() chan *Triple {
 	ch := make(chan *Triple)
 	go func() {
-		for t := range g.triples {
+		for _, t := range g.triples {
 			ch <- t
 		}
 		close(ch)