Creating a new type

Hi,

How can a create a new type or alias?

I want to create a alias for the text type. The alias name must be blob.
(MySQL type).
The documentation tells me how to create en new type, but don't tell me how to
create the input and output functions.

Type definition:
CREATE TYPE blob (INPUT = blob_in, OUTPUT = blob_out, ELEMENT = text);

Regards,
Ferdinand