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