// returns: (action body, closing line number)
// or null
static Pair<S, Int> ctxAction(L<S> lines, int i) {
  while (i <= l(lines) && emptyAfterTrim(get(lines, i))) ++i;
  if (!jmatch("ACTION", get(lines, i))) null;
  int closing = indexOfClosingCurlyBracket(lines, i+1);
  S body = tok_unCurlyBracket(lines(subList(lines, i+1, closing+1)));
  ret pair(body, closing);
}