reserve i,j,k,n for Nat;
reserve D for non empty set,
  p for Element of D,
  f,g for FinSequence of D;

theorem
  for x1,x2 being object holds Rev <*x1,x2*> = <*x2,x1*>
proof
  let x1,x2 be object;
  set f = <*x1,x2*>, g = <*x2,x1*>;
A1: len f = 2 by FINSEQ_1:44;
A2: len g = 2 by FINSEQ_1:44;
  now
    let i;
    assume i in dom g;
    then
A3: i in Seg len f by A1,A2,FINSEQ_1:def 3;
    per cases by A1,A3,FINSEQ_1:2,TARSKI:def 2;
    suppose
A4:   i = 1;
      hence g.i = x2
        .= f.(len f - i + 1) by A1,A4;
    end;
    suppose
A5:   i = 2;
      hence g.i = x1
        .= f.(len f - i + 1) by A1,A5;
    end;
  end;
  hence thesis by A1,A2,Def3;
end;
