Access Control
==============

   The function compute_access returns one of three values:

`access_public'
     means that the field can be accessed by the current lexical scope.

`access_protected'
     means that the field cannot be accessed by the current lexical
     scope because it is protected.

`access_private'
     means that the field cannot be accessed by the current lexical
     scope because it is private.

   DECL_ACCESS is used for access declarations; alter_access creates a
list of types and accesses for a given decl.

   Formerly, DECL_{PUBLIC,PROTECTED,PRIVATE} corresponded to the return
codes of compute_access and were used as a cache for compute_access.
Now they are not used at all.

   TREE_PROTECTED and TREE_PRIVATE are used to record the access levels
granted by the containing class.  BEWARE: TREE_PUBLIC means something
completely unrelated to access control!