boolean (ANSI and GNU Common Lisp Document)

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#### 4.4.2 boolean [Type]

#### Supertypes::

boolean, symbol, t

#### Description::

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

#### Notes::

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.