reserve x,x1,x2,x3,x4,x5,x6,x7,x8,x9,x10,y for object, X,Z for set;

theorem
  for x1,x2,x3 being set holds {x2,x1} \/ {x3,x1} = {x1,x2,x3}
proof
  let x1,x2,x3 be set;
  thus {x2,x1} \/ {x3,x1} = {x2,x1,x3,x1} by Lm2
    .= {x1,x1,x2,x3} by Th69
    .= {x1,x2,x3} by Th31;
end;
