(defun NAME (ARG1 …) [DOCSTRING] EXPR1 …)Define a named lambda-expression, that is: define NAME to evaluate
to the lambda-expression (lambda (ARG1 …) (EXPR1 …))
when called as a function. Also, install DOCSTRING as response to
the commands (help NAME) and (morehelp NAME). Note that
DOCSTRING need not contain the command-synopsis, it is generated
automatically. EXPR1 cannot be a string if DOCSTRING is
omitted; it would be interpreted as the doc-string. The return value of
(defun …) is the function name. Functions can be recursive
and self-modifying. It is possible to redefine builtin-functions, in
this case the old definition is still available under the name
-builtin-OLDNAME-. Argument values may be altered by setq;
the new binding is discarded after evaluation of the surrounding
defun-body. The special keywords &optional and
&rest have the same meaning as for anonymous lambda-expression,
see there. See (lambda …). See (setq …). See (let …).