code: public void setContraction(double contraction)
      {
          if(contraction >= 1 || contraction <= 0 || Double.isNaN(contraction) || Double.isInfinite(contraction) )
              throw new ArithmeticException("Contraction constant must be > 0 and < 1, not " + contraction);
          this.contraction = contraction;
      }
docs: Sets the contraction constant, which must be in the range (0, 1)
  @param contraction the contraction constant