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  ANSI Common Lisp   4 Types and Classes   4.4 Dictionary of Types and Classes

4.4.2 boolean Type

boolean, symbol, t

The type boolean contains the symbols t and nil, which represent true and false, respectively.

See Also:
t (constant variable), nil (constant variable), if, not, complement

Conditional operations, such as if, permit the use of generalized booleans, not just booleans; any non-nil value, not just t, counts as true for a generalized boolean. However, as a matter of convention, the symbol t is considered the canonical value to use even for a generalized boolean when no better choice presents itself.

Allegro CL Implementation Details:

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