GNU Emacs is a self-documenting, customizable, extensible real-time display editor. Users new to Emacs will be able to use basic features fairly rapidly by studying the tutorial and using the self-documentation features. Emacs also has an extensive interactive manual browser. It is easily extensible since its editing commands are written in Lisp. GNU Emacs's many special packages handle mail reading (RMail) and sending (Mail), outline editing (Outline), compiling (Compile), running subshells within Emacs windows (Shell), running a Lisp read-eval-print loop (Lisp-Interaction-Mode), automated psychotherapy (Doctor :-) and many more.