reserve a,b,c for Integer;
reserve i,j,k,l for Nat;
reserve n for Nat;
reserve a,b,c,d,a1,b1,a2,b2,k,l for Integer;

theorem Th6:
  -n is Element of NAT iff n = 0
proof
  thus -n is Element of NAT implies n = 0
  proof
    assume -n is Element of NAT;
    then -n>=0 & n+(-n)>=0+n;
    hence thesis;
  end;
  thus thesis;
end;
