docs: public void setEta(double eta)
      {
          if(Double.isNaN(eta) || eta < 0.5 || eta > 1.0)
              throw new IllegalArgumentException("eta must be in [0.5, 1] not " + eta);
          this.eta = eta;
          this.phi = Normal.invcdf(eta, 0, 1);
          this.phiSqrd = phi*phi;
          this.zeta = 1 + phiSqrd;
          this.psi  = 1 + phiSqrd/2;
      }