Thanks, Thomas and Gukalov!
Looking at "in" in the debugger I see that it is part of the VIP runtime and can be used without having to bring the list class into a project ... which is probably the most trivial of considerations in real life. (It's hard to search for a word (in) that is used so commonly in many non-code ways. )
The 'in' operators are properties (specified as attibutes) of domains and interfaces. So the in operators of a domain/interface are visible when the domain is visible.
The list domain is built-in and always visible, subsequently its in operators are also always visibl.