theorem :: VFUNCT_2:50

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