Implemented override mode as seen in Emacs.

contrib/overwrite-mode/lem-overwrite-mode.asd

@@ -0,0 +1,4 @@
+(defsystem "lem-overwrite-mode"
+  :serial t
+  :depends-on ("lem")
+  :components ((:file "overwrite-mode")))

contrib/overwrite-mode/overwrite-mode.lisp

@@ -0,0 +1,25 @@
+(defpackage :lem-overwrite-mode
+  (:use :cl :lem))
+(in-package :lem-overwrite-mode)
+
+(define-key *global-keymap*
+  "Insert" 'overwrite-mode)
+
+(define-minor-mode overwrite-mode
+    (:name "Overwrite"
+     :global nil
+     :enable-hook 'enable
+     :disable-hook 'disable))
+
+(defun hook-function (char)
+  (declare (ignore char))
+  (unless (end-line-p (current-point))
+    (delete-next-char)))
+
+(defun enable ()
+  (add-hook (variable-value 'self-insert-before-hook)
+            'hook-function))
+
+(defun disable ()
+  (remove-hook (variable-value 'self-insert-before-hook) 
+               'hook-function))