theorem Th7:
  a <= 6*n+1 iff a in <=6n+1(n)
  proof
    thus a <= 6*n+1 implies a in <=6n+1(n);
    assume a in <=6n+1(n);
    then ex b st a = b & b <= 6*n+1;
    hence thesis;
  end;
