for h, x being Real
for f being Function of REAL,REAL st ( for x being Real holds f . x = (cot (#) cos) . x ) & x in dom cot & x - h in dom cot holds
(bD (f,h)) . x = (((1 / (sin x)) - (sin x)) - (1 / (sin (x - h)))) + (sin (x - h))