// a point in a ticker sequence
srecord noeq TickerPoint(TickerSequence ticker, long currentTime) {
  double currentPrice() { ret lookback(0); }
  
  // time = ms to look back
  double lookback(long time) {
    ret ticker.priceAtTimestamp(currentTime-time);
  }
  
  // false if looking back would require going outside of the time range
  bool canLookback(long time) {
    ret currentTime-time >= ticker.startTime();
  }
  
  TickerSequence ticker() { ret ticker; }
  long currentTime aka time() { ret currentTime; }
}