 reserve x for Real,
    p,k,l,m,n,s,h,i,j,k1,t,t1 for Nat,
    X for Subset of REAL;

theorem Th7:
  for i,j being natural Number holds i + j = 0 implies i = 0 & j = 0
proof
  let i,j be natural Number;
  0 <= i & 0 <= j by Th2;
  hence thesis;
end;
