theorem Th39:
  { x1,x1,x1,x2,x3,x4 } = { x1,x2,x3,x4 }
proof
  thus { x1,x1,x1,x2,x3,x4 } = { x1,x1,x2,x3,x4 } by Th33
    .= { x1,x2,x3,x4 } by Th32;
end;
