Set intersection for a whole tree?

How can I get the set intersection of a whole tree ?
In other words, how can I get the items common to all tree branches ?

Tree (3.4 KB)

One way would be to create a set and multiply the number of occurences of each member. If one of the items is not present in a set, the number of occurences is 0. Mass multiplication will also be zero. This can be used to sift the set of unique members

Tree (18.0 KB)

That is very smart !
Thanks Martin