set_name_zero buf f a calls f on buf, with the window adjusted
to where name is. Even though f is given a seekable buffer, it
must move nothing except the lower bound of the window past the data it wrote.
set_name_zero buf f a calls f on buf, with the window adjusted
to where name is. Even though f is given a seekable buffer, it
must move nothing except the lower bound of the window past the data it wrote.