Newbie question about escaping in a function

I have a simple function defined thusly:

CREATE OR REPLACE FUNCTION datemath(timestamp with time zone, int4, varchar)

  RETURNS timestamp AS

'

DECLARE

  tdat timestamp;

  rdat timestamp;

BEGIN

  IF ($1 IS NULL) THEN

    TDAT := NOW();

  ELSE

    TDAT := $1;

  END IF;

  select tdat + interval ''$2 $3'' into rdat;

  return rdat;

END;

'

  LANGUAGE 'plpgsql' VOLATILE;

The problem is the interval part. How do I tell the bugger to use the second and third params as input to interval? I have tried different ways of escaping, from \’$2 $3\’ to ‘’$2 $3’’ and everything else in between, it just doesn’t like it.

Help! J

Thanks,

naeem