3.1 Evaluation 3.1.2 The Evaluation Model 188.8.131.52 Form Evaluation 184.108.40.206.1 Symbols as Forms
220.127.116.11.1.3 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,
a binding for a constant variable,
except that a `compatible' redefinition of a constant variable
using defconstant is permitted; see the macro defconstant.
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.