    A  max (L - rA)       zero-gradient region                  predicted
  1.0       0.0e+00                    r > 1.2                    r > 1.2
 -1.0       0.0e+00                    r < 0.8                    r < 0.8
verified: zero gradient exactly in the predicted region, slope A elsewhere

 step size   fraction of samples with r outside [1-eps,1+eps]   fraction with zero gradient
       0.5                                              0.000                         0.000
       2.0                                              0.000                         0.000
       4.0                                              0.171                         0.171
       8.0                                              0.496                         0.496
      16.0                                              0.833                         0.668
