theorem Th11:
  T deg<= 0 = the set of all x-term
  proof
    thus T deg<= 0 c= the set of all x-term
    proof
      let a; assume a in T deg<= 0;
      then consider r such that
A1:   a = r & deg r <= 0;
A2:   deg @r = deg r = 0 by A1;
      reconsider t = r as Element of Free(S,X) by MSAFREE4:39;
      (ex s,x st t = x-term) or ex o,p st t = o-term p by Th16;
      hence thesis by A1,A2,Th22;
    end;
    let a; assume a in the set of all x-term;
    then consider s,x such that
A3: a = x-term;
    deg (x-term) = 0 <= 0 & x-term in T by Th21,Th24;
    then reconsider r = x-term as Element of T;
    deg r = deg @r = 0 by Th21;
    hence thesis by A3;
  end;
