[tonyg]

Gensym is not enough. There are two kinds of hygiene: avoiding capture by the macro of identifiers used in expressions supplied by the user, and avoiding capture by the user of identifiers supplied by the macro. See http://community.schemewiki.org/?hygiene-versus-gensym and, I guess, https://en.wikipedia.org/wiki/Hygienic_macro#The_hygiene_pro... . (There are surely better presentations of the idea out there, but I can't seem to find them right this minute.)