theorem Th8:
  i1 < 0 implies i1 <= - 1
proof
  assume i1 < 0;
  then 0 < - i1 by XREAL_1:58;
  then 1 <= - i1 by Lm4;
  then - - i1 <= - 1 by XREAL_1:24;
  hence thesis;
end;
