theorem Th11:
  bdif(f,h).1 = bD(f,h)
proof
  bdif(f,h).1 = bdif(f,h).(0+1) .=bD(bdif(f,h).0,h) by DIFF_1:def 7
    .= bD(f,h) by DIFF_1:def 7;
  hence thesis;
end;
