public Variation ComparingTo(BarbellPosition newPosition)
 {
     return(new Variation(previous: current, current: newPosition));
 }
 private Variation(BarbellPosition previous, BarbellPosition current)
 {
     this.previous = previous;
     this.current  = current;
 }
 public static Variation BeginTracking(BarbellPosition startingPosition)
 {
     return(new Variation(null, startingPosition));
 }