theorem Th10c:
  i <= j implies T height<= i c= T height<= j
  proof
    assume Z0: i <= j;
    let a be object;
    assume a in T height<= i;
    then consider t such that
A2: a = t & t in T & height t <= i;
    height t <= j by Z0,A2,XXREAL_0:2;
    hence thesis by A2;
  end;
