sS gazelle_replaceFirstInOfRuleText(S ruleText, S newIn) {
  PairS p = splitAtDoubleArrow_pair(ruleText);
  if (p == null) null;
  LS ins = tok_splitAtPlusAtBeginningOfLine(p.a);
  ins.set(0, newIn);
  ret gazelle_constructRuleText(ins, p.b);
}