// returns: (condition,  goto, closing line number)
// or null
static Triple<S, S, Int> ctxCaseGoto(L<S> lines, int i) {
  new Matches m;
  while (i <= l(lines) && emptyAfterTrim(get(lines, i))) ++i;
  if (!flexMatch("CASE (*) ACTION", get(lines, i), m, false)) null;
  S condition = $1;
  if (!flexMatch("[GOTO] = *", get(lines, i+1), m, false)) null;
  S target = $1;
  int closing = indexOfClosingCurlyBracket(lines, i+2);
  ret triple(condition, target, closing);
}