We can declare that our code only works with lists of numbers, or lists of strings. Therefore, nil is not expected. If (car list) returns nil, it can only be th

We can declare that our code only works with lists of numbers, or lists of strings. Therefore, nil is not expected. If (car list) returns nil, it can only be that the list is empty, because if it were not empty, it would return a number, or string, or widget or whatever the list holds. Even when we have a heterogeneous list in Lisp, like one that can have symbols, numbers, strings or widgets, we can almost always exclude nil as a matter of design, and thus cheerfully use the simpler code. We cannot exclude nil when a list contains Boolean values, because nil is our false. We also cannot exclude it when it contains lists, because nil is our empty list. The beauty is that in many situations, we can arrange not to have to care about the distinction between "item is missing" and "item is false" and "item is an empty list", and then we can write terser code. When you see such terse code from another programmer, you know instinctively what the deal is with how they are treating nil before even looking at any documentation or test cases.