the
difference between using <SID> and <Plug>:

<Plug>	is visible outside of the script.  It is used for mappings which the
	user might want to map a key sequence to.  <Plug> is a special code
	that a typed key will never produce.
	To make it very unlikely that other plugins use the same sequence of
	characters, use this structure: <Plug> scriptname mapname
	In our example the scriptname is "Typecorr" and the mapname is "Add".
	We add a semicolon as the terminator.  This results in
	"<Plug>TypecorrAdd;".  Only the first character of scriptname and
	mapname is uppercase, so that we can see where mapname starts.

<SID>	is the script ID, a unique identifier for a script.
	Internally Vim translates <SID> to "<SNR>123_", where "123" can be any
	number.  Thus a function "<SID>Add()" will have a name "<SNR>11_Add()"
	in one script, and "<SNR>22_Add()" in another.  You can see this if
	you use the ":function" command to get a list of functions.  The
	translation of <SID> in mappings is exactly the same, that's how you
	can call a script-local function from a mapping.


USER COMMAND

Now let's add a user command to add a correction: >

 38	if !exists(":Correct")
 39	  command -nargs=1  Correct  :call s:Add(<q-args>, 0)
 40	endif

The user command is defined only if no command with the same name already
exists.  Otherwise we would get an error here.  Overriding the existing user
command with ":command!" is not a good idea, this would probably make the user
wonder why the command they defined themself doesn't work.  |:command|


SCRIPT VARIABLES

When a variable starts with "s:" it is a script variable.  It can only be used
inside a script.  Outside the script it's not visible.  This avoids trouble
with using the same variable name in different scripts.  The variables will be
kept as long as Vim is running.  And the same variables are used when sourcing
the same script again. |s:var|

The fun is that these variables can also be used in functions, autocommands
and user commands that are defined in the script.  In our example we can add
a few lines to count the number of corrections: >

 19	let s:count = 4
 ..
 30	function s:Add(from, correct)
 ..
 34	  let s:count = s:count + 1
 35	  echo s:count .. " corrections now"
 36	endfunction

First s:count is initialized to 4 in the script itself.  When later the
s:Add() function is called, it increments s:count.  It doesn't matter from
where the function was called, since it has been defined in the script, it
will use the local variables from this script.


THE RESULT

Here is the resulting complete example: >

  1	" Vim global plugin for correcting typing mistakes
  2	" Last Change:	2000 Oct 15
  3	" Maintainer:	Bram Moolenaar <Bram@vim.org>
  4	" License:	This file is placed in the public domain.
  5
  6	if exists("g:loaded_typecorr")
  7	  finish
  8	endif
  9	let g:loaded_typecorr = 1
 10
 11	let s:save_cpo = &cpo
 12	set cpo&vim
 13
 14	iabbrev teh the
 15	iabbrev otehr other
 16	iabbrev wnat want
 17	iabbrev synchronisation
 18		\ synchronization
 19	let s:count = 4
 20
 21	if !hasmapto('<Plug>TypecorrAdd;')
 22	  map <unique> <Leader>a  <Plug>TypecorrAdd;
 23	endif
 24	noremap <unique> <script> <Plug>TypecorrAdd;  <SID>Add
 25
 26	noremenu <script> Plugin.Add\ Correction      <SID>Add
 27
 28	noremap <SID>Add  :call <SID>Add(expand("<cword>"), 1)<CR>
 29
 30	function s:Add(from, correct)
 31	  let to = input("type the correction for " .. a:from .. ": ")
 32	  exe ":iabbrev " .. a:from .. " " .. to
 33	  if a:correct | exe "normal viws\<C-R>\" \b\e" | endif
 34	  let s:count = s:count + 1
 35	  echo s:count .. " corrections now"
 36	endfunction
 37
 38	if !exists(":Correct")
 39	  command -nargs=1  Correct  :call s:Add(<q-args>, 0)
 40	endif
 41
 42	let &cpo = s:save_cpo
 43	unlet s:save_cpo

Line 33 wasn't explained yet.  It applies the new correction to the word under
the cursor.  The |:normal| command is used to use the new abbreviation.  Note
that mappings and abbreviations are expanded