theorem Th85:
  i + k = len f implies Rev(f|k) = Rev f/^i
proof
  assume i + k = len f;
  then
A1: i + k = len Rev f by FINSEQ_5:def 3;
  thus Rev(f|k) = Rev(Rev Rev f |k)
    .= Rev Rev(Rev f/^i) by A1,Th84
    .= Rev f/^i;
end;
