a type of cell. It’s an ordered collection of items that usually share a type. (there are actually some lists that do not share a type but they are uncommon, so keeping it simple, think of lists as an ordered collection of items that do share a type). Note that all lists are null-terminated, and the “null” is the symbol “sig” which is ~. It can be before the list (outside the brackets to the left) or inside the rightmost bracket. A list can be a null-list if they only contain [~].