It is difficult to not block opponent's mill

[calcitem]

It is difficult.

 1.     d2    d6
 2.     b4    f4
 3.     d7    c4
 4.     e4    d1
 5.     f6    a4
 6.     g4    c3
 7.     c5    a1
 8.     g1    g7
 9.     a7   (bad 39)

[calcitem]

 1.     d2    f4
 2.     b4    d6
 3.     g4    d1
 4.     d5    f6
 5.     f2    b6xd2
 6.     d3    d2
 7.     e4    c4
 8.     g7    g1
 9.     a1    a4
10.    g7-d7    a4-a7
11.    d5-e5    c4-c3
12.    d3-e3xc3    d6-d5
13.    d7-d6    (bad 37)

[calcitem]

 1.     b4    d2
 2.     d6    f4
 3.     d3    a4
 4.     g4    d5
 5.     f2    d7
 6.     d1    a7
 7.     g7    g1
 8.     a1    (bad 37， BM Plays e5)

if

1.     b4    d2
 2.     d6    f4
 3.     d3    a4
 4.     g4    d5
 5.     f2    d7
 6.     d1    a7
 7.     g7    g1
 8.     e5    a1xe5
 9.     e5    c3

It is an easy draw now. Just turn the middle ring.

So maybe A1 is played not thinking about mobility far enough. Probably in actual play, a human would have spotted this.

So it's a cascade. Of slightly wrong moves leading to a challenging position. So current analysis is - F2 is ok if followed up correctly. D1 is bad because it doesn't defend a black double attack. And A1 is wrong because it has not considered that the black mill can be easily blocked.

The version v1.1.38 of Sanmill doesn't go D1, it goes A7. But for the latest version, once the above position is encountered, it will still go A1 and perhaps need to raise the weight of mobility. I will test how morris behaves.

[calcitem]

 1.     f4    d6
 2.     d2    b4
 3.     a4    d3
 4.     d7    b6
 5.     b2    f6xd2
 6.     d1    g4
 7.     c5    d2
 8.     c4    c3
 9.     e3    e4
10.    c5-d5    e4-e5
11.    d7-a7    g4-g1
12.    d1-a1xg1    d6-d7
13.    d5-d6    (bad 37)

 1.     d6    f4
 2.     d2    b4
 3.     c4    d7
 4.     a1    g4
 5.     e4    d1
 6.     b6    f6
 7.     f2    b2
 8.     a7    a4  (bad 33)