for U being non empty set
for A1, A2, B1, B2 being Subset of U st A1 c= A2 & B1 c= B2 holds
INTERSECTION ((Inter (A1,A2)),(Inter (B1,B2))) = { C where C is Subset of U : ( A1 /\ B1 c= C & C c= A2 /\ B2 ) }