more helper functions related to nth; alpha release

Author: tiye

README.md

@@ -12,6 +12,8 @@ bisection-key
 
 [![Clojars Project](https://img.shields.io/clojars/v/cirru/bisection-key.svg)](https://clojars.org/cirru/bisection-key)
 
+Under `bisection-key.core`:
+
 ```clojure
 ; make sure the first parameter is littler
 (bisection-key.core/bisect "a" "c") ; "b"
@@ -21,20 +23,29 @@ bisection-key
 bisection-key.core/min-id ; "+"
 bisection-key.core/mid-id ; "T"
 bisection-key.core/max-id ; "z"
+```
 
+Functions under `bisection-key.util`:
+
+```clojure
 ; (def v {"a" 1, "b" 1})
-(bisection-key.util/key-before v "b") ; "aT"
-(bisection-key.util/key-after v "a") ; "aT"
-(bisection-key.util/key-prepend v) ; "G"
-(bisection-key.util/key-append v) ; "n"
-
-(bisection-key.util/assoc-after v "a" 2) ; (assoc v "aT" 2)
-(bisection-key.util/assoc-before v "b" 2) ; (assoc v "aT" 2)
-(bisection-key.util/assoc-prepend v 2) ; (assoc v "G" 2)
-(bisection-key.util/assoc-append v 2) ; (assoc v "n" 2)
-
-(bisection-key.util/get-min-key v) ; "a"
-(bisection-key.util/get-max-key v) ; "b"
+(key-before v "b") ; "aT"
+(key-after v "a") ; "aT"
+(key-prepend v) ; "G"
+(key-append v) ; "n"
+(key-nth v 0) ; "a"
+(val-nth v 0) ; 1
+
+(assoc-after v "a" 2) ; (assoc v "aT" 2)
+(assoc-before v "b" 2) ; (assoc v "aT" 2)
+(assoc-prepend v 2) ; (assoc v "G" 2)
+(assoc-append v 2) ; (assoc v "n" 2)
+(assoc-nth v 0 2) ; (assoc v "a" 2)
+(assoc-before-nth v 1 2) ; (assoc v "aT" 2)
+(assoc-after-nth v 1 2) ; (assoc v "n" 2)
+
+(get-min-key v) ; "a"
+(get-max-key v) ; "b"
 ```
 
 Charset: