Result Function
  * xx means `storage compound_key_reader:result keys[<count>]`
  Arguments:
    c: count
    l,r: substr cur
    key: key name
  Result:
    0: Nothing
    +1: Save substr cur((l,r]) in xx.cur
    +2: Save key name in xx.key
    +4: Save key value in xx.value
    +8: Save the sequence in the keys in xx.seq
    VOID: same as 10