theorem :: VFUNCT_1:50

for X, Y being set
for C being non empty set
for V being RealNormSpace
for f1, f2 being PartFunc of C,V st f1 | X is constant & f2 | Y is constant holds
( (f1 + f2) | (X /\ Y) is constant & (f1 - f2) | (X /\ Y) is constant )