Usage:
x in list
Returns:
1 if x is in list, 0 otherwise
This is a null-safe way of checking if an item is in a list, unlike /list/proc/Find(), which will runtime error if the list is not defined.
To check if an item is not in a given list, you can check !(item in list). This is distinct from checking (!item in list), which would coerce item into 0 or 1, and check if 0 or 1 is in the list.
