[main12_7] return commands added, axis equal lines commented
[main12_7] plot the two derivatives together
[main12_7] Directional Derivative surf
[main12_7] surf of f instead of contour;
a1 > a2 is the default
[main12_7] reordering
[main12_7] default: a1+a2<90
[main12_7] default at zero derivative plot: eps = 0.01
[main12_7a] try: 0 < f* - f
[main12_7b] try: 0 > df/dg1