let h, x be Real; :: thesis: for f being Function of REAL,REAL holds (fD (f,h)) . x = (f . (x + h)) - (f . x)
let f be Function of REAL,REAL; :: thesis: (fD (f,h)) . x = (f . (x + h)) - (f . x)
dom ((Shift (f,h)) - f) = REAL by FUNCT_2:def 1;
hence (fD (f,h)) . x = ((Shift (f,h)) . x) - (f . x) by VALUED_1:13
.= (f . (x + h)) - (f . x) by Def2 ;
:: thesis: verum