// need at least 2 recent candles
static double trueRange(L<TradingCandle> lastCandles) {
  if (l(lastCandles) < 2) ret Double.NaN;
  var c1 = nextToLast(lastCandles);
  var c2 = last(lastCandles);
  ret max(c2.high(), c1.close())-min(c2.low(), c1.close());
}