A tree representation typically represented with parentheses.

When representing source code in Lisp, the first element of an S-expression is commonly an operator or function name and any remaining elements are treated as arguments.

• Lists and pairs: (1 () (2 . 3) (4))
• Symbols: with-hyphen ?@!$ |a symbol with spaces|
• Strings: "Hello, world!"
• Integers: -9876543210
• Floating-point numbers: -0.0 6.28318 6.022e23

Example

(defun factorial (x)
   (if (zerop x)
       1
       (* x (factorial (- x 1)))))

incoming i-expressions