theorem Th3:
  b = 0 or b+a in seq(a,b)
proof
  assume b <> 0;
  then ex c be Nat st b = c + 1 by NAT_1:6;
  then 1 <= b by NAT_1:11;
  then b+a is Element of NAT & 1+a <= b+a by ORDINAL1:def 12,XREAL_1:6;
  hence thesis;
end;
