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Common Lisp the Language, 2nd Edition
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Up: Predicates
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The names nil and t are constants in Common
Lisp. Although they are symbols like any other symbols, and appear to be
treated as variables when evaluated, it is not permitted to modify their
values. See defconstant.
[Constant]
nil
The value of nil is always nil. This object
represents the logical false value and also the empty list. It
can also be written ().
[Constant]
t
The value of t is always t.
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