I'm still not there. I understand the logic of the identity value, but I don't get:
...and this is what is returned for empty lists
What do the other operators (+, *, &&, ||) have to do with empty lists. And why would -∞ be the minimum of an empty list? I'd have guessed undefined as Ilja did.
Further this requirement must only refer to lists (sets) of numbers where -∞ is a valid member.