code: public void setRho(double rho)
      {
          if(rho <= 0 || rho >= 1 || Double.isNaN(rho))
              throw new IllegalArgumentException("Rho must be in (0, 1)");
          this.rho = rho;
      }
docs: Sets the decay rate used by AdaDelta. Lower values focus more on the
  current gradient, where higher values incorporate a longer history.
  
  @param rho the decay rate in (0, 1) to use