**''DEFCONSTANT''**

Define symbol in global scope as constant. Constant symbol cannot be overwritten by ''SETQ'', and cannot be even used as local variable in ''LET''.

''DEFCONSTANT'' form needs two arguments, and can have one more optional description string. The first parameter must be a symbol. Symbols from base package (//reserved//) will not be allowed, but ''DEFCONSTANT'' can be used to overwrite the value of user-defined constant. Second argument is evaluated, and will be bound as the value for the constant. Third, optional argument is a description string.

<code lisp>
>(defconstant eul 2.72 "euler's number")   ; defining new meaningful constant
EUL

>(defconstant pi 3.14)                     ; trying to redefine built-in PI
;Using base-package name for DEFCONSTANT. Not allowed.    ; it fails
NIL

>(defconstant eul 2.71828)                 ; redefine with better precision
EUL
</code>

See also: ''DEFPARAMETER'', ''DEFVAR'', ''SETQ''