Document BEFORE-ELEM.

List-of-Navigators.md

@@ -6,6 +6,7 @@
     - [AFTER-ELEM](#after-elem)
     - [ALL](#all)
     - [ATOM](#atom)
+    - [BEFORE-ELEM](#before-elem)
     - [BEGINNING](#beginning)
     - [DISPENSE](#dispense)
     - [END](#end)
@@ -104,6 +105,15 @@
 2
 ```
 
+## BEFORE-ELEM
+
+`BEFORE-ELEM` navigates to the 'void' element before the sequence. For transformations – if result is not `NONE`, then prepend that value.
+
+```clojure
+=> (setval BEFORE-ELEM 3 [1 2])
+[3 1 2]
+```
+
 ## BEGINNING
 
 `BEGINNING` navigates to the empty subsequence before the beginning of a collection. Useful with `setval` to add values onto the beginning of a sequence.