UNLDPA ( part, ifail )
======================

  Unload part.

Receives:
  KI_tag_part              *part           --- part to unload

Returns:
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_part_not_keyed         part is not keyed and cannot be unloaded

Description:
  UNLDPA removes 'part' from internal memory. The part must be a stored part
  otherwise KI_part_not_keyed is returned in 'ifail'.

  We define the true-sub-parts of some stored (ENSTST) part S as those anonymous
  (ENSTAN) sub-parts of S reachable from S without encountering other stored
  parts.

  The contents of the part, including all anonymous true-sub-parts are deleted
  from internal memory and removed from the world. All associated data of the
  part is also deleted. The part itself remains attached to the world but its
  state is changed to unloaded (ENSTUN).

  The part may be instanced.

  The system is able to determine the key and box of an unloaded part.

  If a later operation requires access to the contents of an unloaded part it
  will be reloaded automatically.