title: convert tree to another tree with new format in Ocaml
body: <p>I want to convert on tree to another tree with different type ,about 1 week i tried the different solution but i didn't successful, please help me.
  my code is :</p>
  
  <pre><code>type (a',b') Tree = Lead of a'| Noeud of b'*a'*(a',b')Tree list
  type strTree = st of int | Leaf of String | Tree of string *Strin* strTree list
  val Tree-strTree :(a'-&gt;string)-&gt;(b'-&gt;string)-&gt;(a',b')Tree -&gt; strTree
  
  let Tree-strTree p pp a =
     let rec mapA a' = match a' with
       | Lead _ -&gt; Tree ("","",[])
       | Noeud (f,r,l)-&gt; Tree(p f ,pp r ,fold_left(fun x y-&gt;x @ [mapA y])[] l)
     in
     mapA  a 
  </code></pre>
  
  <blockquote>
    <p>Error: unbound value fold_left</p>
  </blockquote>
  
  <p>I don't Know how i can use fold_left</p>