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persistable sclass RegularPriceCells is PriceCells {
  // cell size
  settable double interval;
  
  // one of the cell limits
  settable double base;
  
  *(double *interval) {}
  *(double *base, double *interval) {}
  
  double remainder(double price) {
    ret mod(price-base, interval);
  }
  
  // TODO: not sure if this logic has rounding problems
  
  public bool isCellLimit(double price) {
    ret remainder(price) == 0;
  }
  
  public double nextCellLimit(double price) {
    double r = remainder(price);
    ret price+interval-r;
  }
  
  public double previousCellLimit(double price) {
    double r = remainder(price);
    ret price-(r == 0 ? interval : r);
  }
  
  public double nCellLimitsDown(double price, int n) {
    ret previousCellLimit(price)-(n-1)*interval;
  }
  
  public double nCellLimitsUp(double price, int n) {
    ret nextCellLimit(price)+n*interval;
  }
  
  public double priceToCellNumber(double price) {
    ret (price-base)/interval;
  }
  
  public double cellNumberToPrice(double cellNumber) {]
    ret cellNumber*interval+base;
  }
}