theorem Th40:
  { x1,x1,x1,x2,x3,x4,x5 } = { x1,x2,x3,x4,x5 }
proof
  thus { x1,x1,x1,x2,x3,x4,x5 } = { x1,x1,x2,x3,x4,x5 } by Th34
    .= { x1,x2,x3,x4,x5 } by Th33;
end;
