theorem Th9: :: DIFF_2:9

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