val empty : t
empty 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
val registered : t -> string -> bool
registered idx data is
data was registered in the index.