arches Backwards for the first difference between two files.
Argument diff is a difference in bits between the string addresses that have
produced matching signatures. diff is not negative. wn is the word number
in file x where the match was discovered. The matching part
from file y is at least as large an address. sb returns the byte offset within
file x of the byte before which the files differ. */
static b32 sb(sstream * C x, sstream * C y, b32 C diff, b32 C wn)
  {uchar a[512], b[512]; long vbn = wn>>7, indx = wn&(128-1);
   while(vbn>=0){sread(a, x, vbn<<12); sread(b, y, (vbn<<12) + diff);
     {int j; for(j=indx; j>=0; --j) 
     if(((b32 *)a)[j] != ((b32 *)b)[j]) break;
     if(j>=0) {j =4*j+3; while(a[j] == b[j]) --j; 
     return (vbn<<9)+j+1;}