static S ai_ifThenSay(S rule, S input) {
  new Matches m;
  ai_opCheck(rule, "if * then say *", m);
  
  S condition = roundBrackets($1);
  S sf = ai_text(condition, "matches to predicate", "$X");
  if (sf == null) fail("no matching predicate found for " + condition);
  if (!isTrue(ai_callSafeSFOnInput(sf, input))) null;
  ret $2;
}