theorem Th41:
  { x1,x1,x1,x2,x3,x4,x5,x6 } = { x1,x2,x3,x4,x5,x6 }
proof
  thus { x1,x1,x1,x2,x3,x4,x5,x6 } = { x1,x1,x2,x3,x4,x5,x6 } by Th35
    .= { x1,x2,x3,x4,x5,x6 } by Th34;
end;
