// returns: (CtxCase, closing line number)
// or null
static Pair<CtxCase, Int> ctxCase(L<S> lines, int i) {
  new Matches m;
  new CtxCase c;
  while (i <= l(lines) && emptyAfterTrim(get(lines, i))) ++i;
  S l = get(lines, i);
  
  if (startsWith(trim(get(lines, i+1)), ")")) {
    lines.set(i, l += get(lines, i+1));
    lines.remove(i+1);
  }
  
  if (jmatch("DEFAULT ACTION", l) || jmatch("ACTION", l)) {}
  else if (flexMatch("CASE (*) ACTION", l, m, false))
    c.condition = m.get(0);
  else null;
  
  Pair<SS, Int> p = ctxParseProperties(lines, i+1);
  c.properties = p.a;

  int opening = p.b;
  int closing = indexOfClosingCurlyBracket(lines, p.b);
  c.action = tok_unCurlyBracket(lines(subList(lines, p.b, closing+1)));
  ret pair(c, closing);
}

Began life as a copy of #1008883