theorem Th4: :: PDIFF_6:4

for n, m being Nat
for f1, f2 being PartFunc of (REAL m),(REAL n)
for g1, g2 being PartFunc of (REAL-NS m),(REAL-NS n) st f1 = g1 & f2 = g2 holds
f1 + f2 = g1 + g2