ADT's

Please could anyone can help with this apparantly easy question on ADT's


The generic ADT named DIRECTORY(NI), where NI represents any ADT having a NAME operation returning a STRING. An example of NI is the ADT PERSON that has an operation called NAME which returns the name of a person as an instance of type STRING. The operations of the generic directory are as follows:-
CREATEDIR - creates an empty directory.
RETRIEVE - takes a directory and a name(i.e a string) as source and returns an item of NI as its result.
ADD - takes a directory and an item of NI and returns the directory with that item added.
ISINDIR - takes a directory and a name and returns TRUE if an item with that name is in the directory.
COUNT - takes a directory and return the number of items in that directory.

{THE QUESTION}

Using the constructive approach, write down the semantics of the RETRIEVE and COUNT operations.  Use the ADT BSTree as an underlying model.