In Lisp, a parenthesized expression is a function call by default, if not explicitly transformed in some way (for example with `quote`). `(f 1 2 3)` would be wr

In Lisp, a parenthesized expression is a function call by default, if not explicitly transformed in some way (for example with `quote`). `(f 1 2 3)` would be written as `f(1, 2, 3)` in other languages, and `(f)` would be `f()`. So `(-1)` would translate to `(-1)()` which doesn't make sense. Well, not exactly, as for example in CL and Elisp you have a `1+` function[1], which... well, adds 1 to its argument. The difference is that `1+` is not a valid literal number, so it's read as a symbol, and if this symbol has a function attached to it, it is called. In Common Lisp you can work around this by using a special syntax: `|-1|`, which forces reading the thing inside as a symbol. You can then define functions and call them this way: (defun |+1| (x) (1+ x)) (|+1| 1) ;; => 2 [1] `1-` too, of course.