for X, Y being non empty set
for x, y being set
for E1, E2 being Subset of [:X,Y:] st E1 misses E2 holds
( (chi ((E1 \/ E2),[:X,Y:])) . (x,y) = ((chi ((X-section (E1,x)),Y)) . y) + ((chi ((X-section (E2,x)),Y)) . y) & (chi ((E1 \/ E2),[:X,Y:])) . (x,y) = ((chi ((Y-section (E1,y)),X)) . x) + ((chi ((Y-section (E2,y)),X)) . x) )