libpq - getting value of SERIAL field when INSERT-ing?

Using libpq, how do I get the return value from an INSERT statement?
 
CREATE TABLE foo ( pk SERIAL , val text, PRIMARY KEY (pk) );
INSERT INTO foo VALUES ( DEFAULT, 'stuff' ) RETURNING (pk);
 
How do I get the value assigned to 'pk'?  I assume it's in the 'PGresult' returned from 'PQexec()', but I can't find anything in the documentation for 'PGresult' that appears to discuss this particular use.

Regards,

- Roy

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