theorem Th50:
  1 <= i & i <= j & j < m & m <= n & n <= len f & (1 < i or n <
  len f) implies L~mid(f,j,i) misses L~mid(f,m,n)
proof
  mid(f,i,j) = Rev mid(f,j,i) by FINSEQ_6:196;
  then L~mid(f,i,j) = L~mid(f,j,i) by SPPOL_2:22;
  hence thesis by Th47;
end;
