The comma means to use the value of place there, instead of the literal symbol 'place'. (Ironically, MJD made one of the same mistakes there he complained abou

The comma means to use the value of place there, instead of the literal symbol 'place'. (Ironically, MJD made one of the same mistakes there he complained about in his C examples -- mentioning 'v' twice, which makes any side effects happen twice. Lisp macros are better than C's, but still require some care.) defsetf turns that expression into whatever the defsetf expander for f1 returns -- it's up to the code.