// position = 1 to 3
static Symbol ai_getTriplePart(S tripleID, int position) {
  TripleWeb t = ai_getTriple(tripleID);
  ret t == null ? null : tripleGet(t, position-1);
}

// e.g. "bdmvnpnkmkoecjyt/a"
static Symbol ai_getTriplePart(S triplePartID) {
  L<S> l = splitAtSlash(triplePartID);
  if (l(l) != 2) null;
  S tripleID = first(l), pos = second(l);
  int iPos = charDiff(lower(pos), 'a');
  TripleWeb t = ai_getTriple(tripleID);
  ret t == null ? null : tripleGet(t, iPos);
}