Prefer alter-var-root over def

Author: bbatsov

README.md

@@ -529,6 +529,23 @@ pairwise constructs as found in e.g. `let` and `cond`.
  ...)
  ```
 
+* <a name="alter-var"></a>
+ Use `alter-var-root` instead of `def` to change the value of a var.
+<sup>[[link]](#alter-var)</sup>
+
+ ```Clojure
+ ;; good
+ (def thing 1) ; value of thing is now 1
+ ; do some stuff with thing
+ (alter-var-root #'thing (constantly nil)) ; value of thing is now nil
+
+ ;; bad
+ (def thing 1)
+ ; do some stuff with thing
+ (def thing nil)
+ ; value of thing is now nil
+ ```
+
 * <a name="nil-punning"></a>
  Use `seq` as a terminating condition to test whether a sequence is
  empty (this technique is sometimes called *nil punning*).