diff --git a/include/privilege.h b/include/privilege.h
index b033839..4eb7564 100644
--- a/include/privilege.h
+++ b/include/privilege.h
@@ -40,6 +40,7 @@ struct PrivilegeSet {
 
 int privilegeset_in_set(struct PrivilegeSet *set, const char *priv);
 struct PrivilegeSet *privilegeset_set_new(const char *name, const char *privs, PrivilegeFlags flags);
+struct PrivilegeSet *privilegeset_extend(struct PrivilegeSet *parent, const char *name, const char *privs, PrivilegeFlags flags);
 struct PrivilegeSet *privilegeset_get(const char *name);
 void privilegeset_ref(struct PrivilegeSet *set);
 void privilegeset_unref(struct PrivilegeSet *set);
diff --git a/src/privilege.c b/src/privilege.c
index 91cc39d..c2ce6e2 100644
--- a/src/privilege.c
+++ b/src/privilege.c
@@ -40,6 +40,8 @@ privilegeset_set_new(const char *name, const char *privs, PrivilegeFlags flags)
 {
 	struct PrivilegeSet *set;
 
+	s_assert(privilegeset_get(name) == NULL);
+
 	set = rb_malloc(sizeof(struct PrivilegeSet));
 	set->refs = 1;
 	set->name = rb_strdup(name);
@@ -51,6 +53,28 @@ privilegeset_set_new(const char *name, const char *privs, PrivilegeFlags flags)
 	return set;
 }
 
+struct PrivilegeSet *
+privilegeset_extend(struct PrivilegeSet *parent, const char *name, const char *privs, PrivilegeFlags flags)
+{
+	struct PrivilegeSet *set;
+
+	s_assert(parent != NULL);
+	s_assert(name != NULL);
+	s_assert(privs != NULL);
+	s_assert(privilegeset_get(name) == NULL);
+
+	set = rb_malloc(sizeof(struct PrivilegeSet));
+	set->refs = 1;
+	set->name = rb_strdup(name);
+	set->flags = flags;
+	set->privs = rb_malloc(strlen(parent->privs) + 1 + strlen(privs) + 1);
+	strcpy(parent->privs, set->privs);
+	strcat(set->privs, " ");
+	strcat(set->privs, privs);
+
+	return set;
+}
+
 struct PrivilegeSet *
 privilegeset_get(const char *name)
 {