Common Lisp has a global variable called read-base ,which controls also base numbers are read in with. This, IIRC, was put in because some earlier Lisps read nu

Common Lisp has a global variable called read-base ,which controls also base numbers are read in with. This, IIRC, was put in because some earlier Lisps read numbers in octal, while others used decimal. Setting this variable to something higher than 10 can give you strange results. For example, nil => NIL (eql '() nil) => T (setf *read-base* 36) nil => 30477 (eql '() nil) ERROR: 19101 is not a function name; try using a symbol instead (|EQL| '() nil) => NIL For you non-lispers, the empty list '() is equal to the "false" value, nil. Also, the pipes are special syntax to allow for symbols which normally not parse as symbols. (The uppercase is for historical reasons too). The moral of this story: "See that control variable over there, the one that does weird things to parsing? Don't touch it unless you know what you're doing."