#	@(#)structures	5.2 (Berkeley) 11/1/93

There are three major data structures in this package.  The first is a
single global structure (named GS) which contains information common to
all files and screens.  It's really pretty tiny, and functions more as a
single place to hang things than anything else.

The second and third structures are the file structures (named EXF) and
the screen structures (named SCR).  They contain information theoretically
unique to a screen or file, respectively.  Each SCR structure has a set
of functions which update the screen and/or return information about the
screen from the underlying screen package.

The GS structure contains linked lists SCR structures.  The structures
can also be classed by persistence.  The GS structure never goes away
and the SCR structure persists over instances of files.

File names have different properties than files themselves, so the name
information for a file is held in an FREF structure which is chained from
the SCR structure.

In general, functions are always passed an SCR structure and often an EXF
structure as well.  The SCR structure is necessary for any routine that
wishes to talk to the screen, the EXF structure is necessary for any
routine that wants to modify the file.  The relationship between an SCR
structure and its underlying EXF structure is not fixed, and although you
can translate from an SCR to the underlying EXF, it is discouraged.  If
this becomes too onerous, I suspect I'll just stop passing around the EXF
in the future.

The naming of the structures is consistent across the program.  (Macros
even depend on it, so don't try and change it!)  The global structure is
"gp", the screen structure is "sp", and the file structure is "ep".

A few other data structures:

TEXT	In nvi/cut.h.  This structure describes a portion of a line,
	and is used by the input routines and as the "line" part of a
	cut buffer.

CB	In nvi/cut.h.	A cut buffer.  A cut buffer is a place to
	hang a list of TEXT structures.

MARK	In nvi/mark.h.  A cursor position, consisting of a line number
	and a column number.

MSG	In nvi/msg.h.  A chain of messages for the user.

SEQ	In nvi/seq.h.  An abbreviation or a map entry.

EXCMDARG
	In nvi/ex/excmd.h.stub.  The structure that gets passed around
	to the functions that implement the ex commands.  (The main
	ex command loop (see nvi/ex/ex.c) builds this up and then passes
	it to the ex functions.)

VICMDARG
	In nvi/vi/vcmd.h.  The structure that gets passed around to the
	functions that implement the vi commands.  (The main vi command
	loop (see nvi/vi/vi.c) builds this up and then passes it to the
	vi functions.)