reserve i,n,m,k,x for Nat,
  i1,i2 for Integer;

theorem Th1:
  i1 in k-SD_Sub implies -Radix(k-'1) - 1 <= i1 & i1 <= Radix(k-'1)
proof
  assume i1 in k-SD_Sub;
  then
  ex i be Element of INT st i = i1 & -Radix(k-'1) - 1 <= i & i <= Radix(k-' 1);
  hence thesis;
end;
