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