A union or an intersection of a family of sets indexed by a name ranging over a
set: union(A_i, i in I) is ⋃_{i ∈ I} A_i, the set of what is in some
A_i, and intersection(A_i, i in I) is ⋂_{i ∈ I} A_i, what is in
every one. A binder, like a quantifier: the name is bound throughout the body and
the index set. Folded to the binary operators over a listed index set -- the
reference's ⋃_{i=1}^{10} {i, 2i} is union({i, 2 i}, i in ZZ+ /\ [1; 10]) -- and an object that answers membership through the quantifiers otherwise:
x in union(A_i, i in I) is exists i in I : x in A_i.
https://github.com/asc-community/AngouriMath/issues/1409