theorem Th65:
  { x1,x2,x3,x4 } = { x2,x1,x3,x4 }
proof
  thus { x1,x2,x3,x4 } = { x1,x2,x3 } \/ { x4 } by Th6
    .= { x2,x1,x3 } \/ { x4 } by Th58
    .= { x2,x1,x3,x4 } by Th6;
end;
