An identifier can be used as the name of a function or variable. In nearly all languages, an identifier can begin with a letter, followed by any combination of letters or numbers. In Scheme, the following characters can also be used, provided that the front of an identifier does not look like a number.
+ - . * / < = > ! ? : % _ ~ ^
The following identifiers are keywords and should not be
redefined by the user.
=> do or
and else quasiquote
begin if quote
case lambda set!
cond let unquote
define let* unquote-splicing
delay letrec
The user also should not redefine names of system functions
such as +, write, etc.