theorem Th10b:
  i <= j implies T deg<= i c= T deg<= j
  proof
    assume Z0: i <= j;
    let a be object;
    assume a in T deg<= i;
    then consider r such that
A2: a = r & deg r <= i;
    deg r <= j by Z0,A2,XXREAL_0:2;
    hence thesis by A2;
  end;
