reserve A,B,O for Ordinal,
        o for object,
        x,y,z for Surreal,
        n,m for Nat;

theorem Th16:
  x = [{y},{}] & y < 0_No implies x == 0_No
proof
  assume x=[{y},{}] & y < 0_No;
  then
A1: L_x<<{0_No}<< R_x by SURREALO:21;
  born 0_No ={} by SURREAL0:37;
  then for z st L_x << {z} << R_x holds born 0_No c= born z;
  hence thesis by SURREALO:16,A1;
end;
