static AI_Rule ai_parseRule(S s) {
  L<S> tok = tok_combineCurlyBraces(javaTok(s));
  new Matches m;
  if (jmatch("* => *", tok, m))
    ret ai_rule(
      ai_parseTriple(m.get(0)),
      ai_parseTriple(m.get(1)));
  if (jmatch("* & * => *", tok, m))
    ret ai_rule(
      ai_parseTriple(m.get(0)),
      ai_parseTriple(m.get(1)),
      ai_parseTriple(m.get(2)));
  null;
}