How does Lisp let you redefine the language itself?

Question

I\'ve heard that Lisp lets you redefine the language itself, and I have tried to research it, but there is no clear explanation anywhere. Does anyone have a simple example?<

Answer 1

Cool example at http://www.cs.colorado.edu/~ralex/papers/PDF/X-expressions.pdf

reader macros define X-expressions to coexist with S-expressions, e.g.,

? (cx ) 
62

plain vanilla Common Lisp at http://www.AgentSheets.com/lisp/XMLisp/XMLisp.lisp ...

(eval-when (:compile-toplevel :load-toplevel :execute)
  (when (and (not (boundp '*Non-XMLISP-Readtable*)) (get-macro-character #\<))
    (warn "~%XMLisp: The current *readtable* already contains a #/< reader function: ~A" (get-macro-character #\<))))

... of course the XML parser is not so simple but hooking it into the lisp reader is.