thanks but can you explain that if x belongs to S1 this means that x belongs to U(S1)? S1 is a set of sets and U(S1) is a set of elements which are in the sets embedded in S1, so S1 doesn't exist in U(S1) since U(S1) doesn't contain sets? I'm probably over-complicating it since it's obvious why if x belongs so a set Y then x also belongs to u(Y) but the capital U makes it strange. Also why do you introduce A and B, if they just translate to S1 and S2 respectively? Is this a convention?