Persistent.MakeMake(Key) creates an index with the given Key.
val empty : tempty an empty index
string idx key returns data associated with the provided key. Returns an empty string if there are no data.
key idx data returns the key associated with data, if no key is associated then Key.null is returned
register idx data registers data in the index idx. If data is already registered, then the index is returned unchanged. Otherwise returns an index where a fresh new key is associated with data.
val registered : t -> string -> boolregistered idx data is true if data was registered in the index.