http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/259-guard-selection.html
From this, the client will hash some information
about themselves [XXX what info should we use? —isis] and, from that,
choose #P number of points on the ring, where #P is
{UTOPIC,DYSTOPIC}_GUARDLIST_ATTEMPTED_THRESHOLD proportion of the
total number of unique relays inserted (if a duplicate is selected, it
is discarded).