reserve n for non zero Nat,
  j,k,l,m for Nat,
  g,h,i for Integer;

theorem
  for n be non zero Nat st h + i <= 2 to_power (n-'1) - 1 & h >= 0 & i
  >= 0 holds Intval(2sComplement(n,h) + 2sComplement(n,i)) = h+i
proof
  let n be non zero Nat such that
A1: h + i <= 2 to_power (n-'1) - 1 and
A2: h >= 0 & i >= 0;
  reconsider h,i as Element of NAT by A2,INT_1:3;
A3: 2sComplement(n,i) = n-BinarySequence(|.i.|) by Def2
    .= n-BinarySequence(i) by ABSVALUE:def 1;
  2sComplement(n,h) = n-BinarySequence(|.h.|) by Def2
    .= n-BinarySequence(h) by ABSVALUE:def 1;
  hence thesis by A1,A3,Th14;
end;
