ref: cea9e2267a70d17173c2b0fcdc421ee11b66ee79
dir: /sys/src/ape/cmd/diff/diagmeet.note/
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