Custom conversion

Hi,

How can I make custom conversion? The postgres-documentation can't help
me :(

I'm find this only, but without any examples:

http://www.postgresql.org/docs/8.3/interactive/sql-createconversion.html



CREATE [ DEFAULT ] CONVERSION name
     FOR source_encoding TO dest_encoding FROM funcname


The function must have the following signature:
conv_proc(
     integer,  -- source encoding ID
     integer,  -- destination encoding ID
     cstring,  -- source string (null terminated C string)
     internal, -- destination (fill with a null terminated C string)
     integer   -- source string length
) RETURNS void;


How can I make this conv_proc function?


Thank's

--
The best regards
Yuri Gordienko