Constant Variables (ANSI and GNU Common Lisp Document)

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#### 3.1.2.5 Constant Variables

Certain variables, called constant variables, are reserved as “named constants.” The consequences are undefined if an attempt is made to assign a value to, or create a binding for a constant variable, except that a ‘compatible’ redefinition of a constant variable using defconstant is permitted; see the macro defconstant.

Keywords, symbols defined by Common Lisp or the implementation as constant (such as nil, t, and pi), and symbols declared as constant using defconstant are constant variables.