def bst_search(root, value):
  """
  Search a binary search tree for a value.

  """
  if root.value == value:
    return True
  if root.value > value:
    return False if root.left is None \
        else bst_search(root.left, value)
  return False if root.right is None \
      else bst_search(root.right, value)