Extending PostGreSQL/PostGIS with an abstract data type.

Dear all,

I have a question concerning "an extension of Postresql/PostGIS with a
spatio-temporal Abstract data type".

PostgreSQL have many types
categories : base types, composite types, domains, etc...

I would like to extend PostGIS type system with a new abstract
data type.

This means that i must guaranty encapsulation of attributes :
which means normally any user of a the DBMS may use the new type through
it's operations and can not select over the types attributes (like
geometry, line or point types)

My question is : can this abstract data type be simply a compsite
type (created whith CREATE TYPE and defined with plpgSQL) or must it be a
base type (thus created inevitabely with a low level langage like C with typedef )?

When i tried with a composite type, i could select over my type
attributes! and it's not an abstract data type any more!

And if it's defined with "typedef", must all operations be defined also in C or can some functions be defined with plpgsql?

Thank
you for you collaboration

Donia