theorem Th1:
  n1 > n or n2 > n implies n1 + n2 > n
proof
A1: n1 > n & n2 >= 0 implies n1 + n2 > n + 0 by XREAL_1:8;
A2: n1 >= 0 & n2 > n implies n1 + n2 > n + 0 by XREAL_1:8;
  assume n1 > n or n2 > n;
  hence thesis by A1,A2;
end;
