--- a/emacs.rst	Sun Aug 01 00:08:02 2010 +0300
+++ b/emacs.rst	Sun Aug 01 13:32:07 2010 +0300
@@ -99,6 +99,23 @@
 With transient-mark-mode and delete-selection-mode enabled: select region,
 type M-| sort -u RET to replace selection with sorted and uniquified lines.
 
+** Determining running environment.
+
+Check such variable:
+
+  window-system - 'nil' if in terminal, 'w32' if native Windows build, 'x' if under X Window.
+  window-system-version
+  emacs-major-version
+  emacs-minor-version
+
+Run such checks:
+
+  (when window-system ...)
+  (when (eq window-system 'x) ...)
+  (when (>= emacs-major-version 22) ...)
+  (when (fboundp '...) ...)
+  (when (featurep '...) ...)
+
 * Finding emacs packages.
 
 See
@@ -181,3 +198,8 @@
 ** Debian.
 
   $ apt-get install emacs
+
+* Emacs paths.
+
+  source-directory data-directory doc-directory exec-directory invocation-directory trash-directory
+  tutorial-directory user-emacs-directory widget-image-directory