title: find the node with minimal value in binary tree with foldr
body: <p>Someone can help me to define a function minTree, that find the node with minimum value in a binary tree using foldr.
  I start something, but I don't know how to use foldr within my code:</p>
  <pre><code>data Tree a 
      = Leaf
      | Node (Tree a) a (Tree a)
      deriving (Show, Eq)
  
    minTree :: (Ord a) =&gt; Tree a -&gt; Maybe a
    minTree Leaf = Nothing
    minTree (Node Leaf a _) = Just a
    minTree (Node left a _) = minTree left
    minTree = foldr...
  </code></pre>
  <p>I am stuck on how define foldr to return the minimum Node</p>