Foreign keys and pg_user table

Dear All,

I'm trying to create a table where the username and email can only be
inserted into the table if the username is already in pg_user. The method
I'm trying is:

CREATE user(
  usename name,
  email text,
  FOREIGN KEY usename REFERENCES (pg_user)
);

but I get told that "...pg_user is not a table."

Is there another way of doing what I want?

Many thanks

Colin

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