How do I use a binary operator (such as union or direct sum) as a large operator (such as sum or product)?
First of all, you need to define the desired symbol as a personal user symbol, as describe in the FAQ How do I use a symbol not provided in Math's Catalog of characters?.
For instance, if you named union symbol
%union, you should enter:
oper %union from i in I A_i
to get the following formula:
All symbols defined by user can be used as large operator if they are preceded with
oper, like in previous example.