Add set length to Util

src/Util.md

@@ -4,6 +4,10 @@ This module contains functions that extend the functionality of standard data ty
 
 ```idris
 module Util
+
+import Data.SortedSet
+
+%default total
 ```
 
 ## Either
@@ -45,3 +49,20 @@ namespace Pair
   (x_1, x_2) >-< (y_1, y_2) = (x_1 - y_1, x_2 - y_2)
 ```
 
+## Set
+
+Extend `Data.SortedSet`
+
+<!-- idris
+namespace Set
+-->
+
+### length
+
+Count the number of elements in a sorted set
+
+```idris
+  export
+  length : SortedSet k -> Nat
+  length x = foldl (\acc, e => acc + 1) 0 x
+```