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