theorem :: VFUNCT_2:30

for M being non empty set
for V being ComplexNormSpace
for f1, f2 being PartFunc of M,V
for X being set holds
( (f1 - f2) | X = (f1 | X) - (f2 | X) & (f1 - f2) | X = (f1 | X) - f2 & (f1 - f2) | X = f1 - (f2 | X) )