:: deftheorem defines closed_wrt_A6 ZF_FUND1:def 11 :
for V being Universe
for X being Subclass of V holds
( X is closed_wrt_A6 iff for a, b being Element of V st a in X & b in X holds
{ (x \ y) where x, y is Element of V : ( x in a & y in b ) } in X );