The syntax about CREATE DOMAIN

Dear Sir,
I would like to tell you that the SQL about "Create Domain" is still
cannot involve constraints other than NULL | NOT NULL in Postgresql 7.4.1.

For instances, when I type the following sentences, it shows error
message:

Create Domain GenderDomain CHAR(1) Constraint g Check Value in ('M','F');

Error: syntax error at or new "VALUE" at character ...

How can I obtain the correct syntax about domain in postgresql?

Regards,
Gary

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