for T1, T2 being non empty TopSpace
for B1 being prebasis of T1
for B2 being prebasis of T2 holds { [: the carrier of T1,b:] where b is Subset of T2 : b in B2 } \/ { [:a, the carrier of T2:] where a is Subset of T1 : a in B1 } is prebasis of [:T1,T2:]