reserve x,y,z for set;
reserve f,f1,f2,f3 for FinSequence,
  p,p1,p2,p3 for set,
  i,k for Nat;
reserve D for non empty set,
  p,p1,p2,p3 for Element of D,
  f,f1,f2 for FinSequence of D;

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;
