some improvements from integrating into go-ipfs

set.go

+package cid
+
+type Set struct {
+	set map[string]struct{}
+}
+
+func NewSet() *Set {
+	return &Set{set: make(map[string]struct{})}
+}
+
+func (s *Set) Add(c *Cid) {
+	s.set[string(c.Bytes())] = struct{}{}
+}
+
+func (s *Set) Has(c *Cid) bool {
+	_, ok := s.set[string(c.Bytes())]
+	return ok
+}
+
+func (s *Set) Remove(c *Cid) {
+	delete(s.set, string(c.Bytes()))
+}
+
+func (s *Set) Len() int {
+	return len(s.set)
+}
+
+func (s *Set) Keys() []*Cid {
+	var out []*Cid
+	for k, _ := range s.set {
+		c, _ := Cast([]byte(k))
+		out = append(out, c)
+	}
+	return out
+}