Here is a comparison matrix which shows a case in which
it is possible for the forward and backward scan in `diag'
to meet along a nonzero length of diagonal simultaneous
(so that bdiag[d] and fdiag[d] are not equal)
even though there is no snake on that diagonal at the meeting point.
85 1 1 1 159 1 1 17
1 2 3 4
60
1 2
1
2 2 3 4
71
3 3 4 5
85
4 3 4 5
17
5 4 5
1
6 4 5 6
183
7 5 6 7
10
8 6 7
1
9 6 7 8
12
7 8 9 10
13
10 8 9 10
14
10 9 10
17
10 10
1
10 9 10
1
8 10 10 10
183
8 7 9 9 9
10
7 6 8 9 8 8
1
6 5 7 7
1
5 6 6
1
5 5 5
50
5 4 4 4
1
4 3 3
85
5 4 3 2 2
1
2 1
17
5 4 3 2 1 1
1
1 0
85 1 1 1 159 1 1 17