theorem
  for k st 1 <= k & k+1 <= len f holds Int right_cell(f,k) c= RightComp f
proof
  let k such that
A1: 1 <= k and
A2: k+1 <= len f;
A3: len f = len Rev f by FINSEQ_5:def 3;
  k <= len f by A2,NAT_1:13;
  then
A4: len f-'k+k = len f by XREAL_1:235;
  then
A5: 1 <= len f-'k by A2,XREAL_1:6;
A6: len f-'k+1 <= len f by A1,A4,XREAL_1:6;
A7: right_cell(f,k) = left_cell(Rev f,len f-'k) by A1,A4,A5,Th9;
  RightComp f = LeftComp Rev f by Th21;
  hence thesis by A3,A5,A6,A7,Th19;
end;
