Help with SET TRANSACTION in a function

Hello,

I was wondering if anyone here can help.  I could not get any help on
pgsql-sql  nor pgsql-general mailing lists.

I'm having trouble getting functions with SET TRANSACTION ISOLATION
... to parse.

This is the error I'm getting (example function below the error):

=> select simple_fun();
NOTICE:  plpgsql: ERROR during compile of simple_fun near line 3
ERROR:parse error at or near ";"

This is the simple_fun function that is causing the above error:

CREATE FUNCTION simple_fun() RETURNS INTEGER AS '
BEGIN
  SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
  BEGIN;
  RETURN 1;
  END;
END;
' LANGUAGE 'plpgsql';

This is as simple as it gets.
I think my syntax is correct (I checked Practical PostgreSQL
book as well as a number of 7.2 PDF documents, etc.).

Am I missing a secret ingredient here?

Thank you,
Otis