reserve k,m,n for Nat,
  i1,i2,i3 for Integer,
  e for set;
reserve i,k,m,n,p,x,y for Nat;

theorem Th7:
  for e be object holds e in 0-SD iff e = 0
proof
  let e be object;
A1: Radix(0) = 1 by POWER:24;
  hereby
    assume e in 0-SD;
    then ex b be Element of INT st e = b & b <= 0 & b >= 0 by A1;
    hence e = 0;
  end;
  assume
A2: e = 0;
  then e is Element of INT by INT_1:def 2;
  hence thesis by A1,A2;
end;
