Update stack lts

src/Dodge/Tree/GenerateStructure.hs

@@ -6,6 +6,7 @@ A /trunk/ refers to the successive first nodes in lists of children.
 module Dodge.Tree.GenerateStructure
     where
 import RandomHelp
+import Control.Monad
 
 import Data.Tree
 {- Single branched tree (a trunk).  -}