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