I want to access a particular element of this disjoint union. (For example I need to check whether an given element is of A or of B.)
What is a good informal notation for this?
Computer proof assistants use data structures to represent disjoint union, with symbolic field names (like structures in programming languages). But in informal mathematics (following ZFC pattern) using identifiers is not customary. Instead they refer to element 0 for A and 1 for B. This is not aligned well with human perception and intuition.
How would you advise to denote this? (without going too formal)