The key part of a catalog header file is a C structure definition
   describing the layout of each row of the catalog.  This begins with
   a CATALOG macro, which so far as the C compiler is
   concerned is just shorthand for typedef struct
   FormData_.
   Each field in the struct gives rise to a catalog column.
   Fields can be annotated using the BKI property macros described
   in catalognamegenbki.h, for example to define a default value
   for a field or mark it as nullable or not nullable.
   The CATALOG line can also be annotated, with some
   other BKI property macros described in genbki.h, to
   define other properties of the catalog as a whole, such as whether
   it is a shared relation.
  
   The system catalog cache code (and most catalog-munging code in general)
   assumes that the fixed-length portions of all system catalog tuples are
   in fact present, because it maps this C struct declaration onto them.
   Thus, all variable-length fields and nullable fields must be placed at
   the end, and they cannot be accessed as struct fields.
   For example, if you tried to
   set pg_type.typrelid
   to be NULL, it would fail when some piece of code tried to reference
   typetup->typrelid (or worse,
   typetup->typelem, because that follows
   typrelid).  This would result in
   random errors or even segmentation violations.
  
   As a partial guard against this type of error, variable-length or
   nullable fields should not be made directly visible to the C compiler.
   This is accomplished by wrapping them in #ifdef
   CATALOG_VARLEN ... #endif (where
   CATALOG_VARLEN is a symbol that is never defined).
   This prevents C code from carelessly trying to access fields that might
   not be there or might be at some other offset.
   As an independent guard against creating incorrect rows, we
   require all columns that should be non-nullable to be marked so
   in pg_attribute.  The bootstrap code will
   automatically mark catalog columns as NOT NULL
   if they are fixed-width and are not preceded by any nullable or
   variable-width column.
   Where this rule is inadequate, you can force correct marking by using
   BKI_FORCE_NOT_NULL
   and BKI_FORCE_NULL annotations as needed.
  
   Frontend code should not include any pg_xxx.h
   catalog header file, as these files may contain C code that won't compile
   outside the backend.  (Typically, that happens because these files also
   contain declarations for functions
   in src/backend/catalog/ files.)
   Instead, frontend code may include the corresponding
   generated pg_xxx_d.h header, which will contain
   OID #defines and any other data that might be of use
   on the client side.  If you want macros or other code in a catalog header
   to be visible to frontend code, write #ifdef
   EXPOSE_TO_CLIENT_CODE ... #endif around that
   section to instruct genbki.pl to copy that section
   to the pg_xxx_d.h header.
  
   A few of the catalogs are so fundamental that they can't even be created
   by the BKI create command that's
   used for most catalogs, because that command needs to write information
   into these catalogs to describe the new catalog.  These are
   called bootstrap catalogs, and defining one takes
   a lot of extra work: you have to manually prepare appropriate entries for
   them in the pre-loaded contents of pg_class
   and pg_type, and those entries will need to be
   updated for subsequent changes to the catalog's structure.
   (Bootstrap catalogs also need pre-loaded entries
   in pg_attribute, but
   fortunately genbki.pl handles that chore nowadays.)
   Avoid making new catalogs be bootstrap catalogs if at all possible.