Data hiding and type safety

Hi all,

  I am just interested what is the Pascal way  of  dealing  with
  (type) safety and data hiding.

  For example, I am  writing  a  module  that  handles  threaded
  trees. What should it export?

  If I export the definiton of my tree type,  I'll  disclose  my
  internals, which is not safe (no data hiding);  on  the  other
  hand, if I don't do it, the interface functions will  have  to
  operate with an index or a pointer to indentify a tree  inside
  my module, which is not type safe...

  Doesn't Extended Pascal deal with such  problems  by  offering
  restricted types?

Thanks in advance,
Anton