theorem Th53:
  { x1,x1,x1,x1,x1,x1,x2,x3 } = { x1,x2,x3 }
proof
  thus { x1,x1,x1,x1,x1,x1,x2,x3 } = { x1,x1,x2,x3 } by Th50
    .= { x1,x2,x3 } by Th31;
end;
