for x0, x1, x2, x3, x4, x5 being Real
for f1, f2 being Function of REAL,REAL holds [!(f1 + f2),x0,x1,x2,x3,x4,x5!] = [!f1,x0,x1,x2,x3,x4,x5!] + [!f2,x0,x1,x2,x3,x4,x5!]