+ * some kind of sorted type, example:
+ ZADD mykey foo 100
+ ZADD mykey bar 50
+ ZRANGE mykey 0 1 => bar foo
+ This is able to take elements sorted because a binary tree is used to store
+ the elements by 'score', with the actual value being the key. On the other
+ side the type also takes an hash table with key->score mapping, so that when
+ there is an update we lookup the current score and can traverse the tree.
+ * BITMAP type