code: def validate(self):
          
          has_more_nodes = True
          visited = set()
          to_visit = [self]
          index = 0
  
          while has_more_nodes:
              has_more_nodes = False
              next_nodes = []
  
              for node in to_visit:
                  if node is None:
                      next_nodes.extend((None, None))
                  else:
                      if node in visited:
                          raise NodeReferenceError(
                              'cyclic node reference at index {}'.format(index))
                      if not isinstance(node, Node):
                          raise NodeTypeError(
                              'invalid node instance at index {}'.format(index))
                      if not isinstance(node.value, numbers.Number):
                          raise NodeValueError(
                              'invalid node value at index {}'.format(index))
                      if node.left is not None or node.right is not None:
                          has_more_nodes = True
                      visited.add(node)
                      next_nodes.extend((node.left, node.right))
                  index += 1
  
              to_visit = next_nodes
docs: Check if the binary tree is malformed.
  
          :raise binarytree.exceptions.NodeReferenceError: If there is a
              cyclic reference to a node in the binary tree.
          :raise binarytree.exceptions.NodeTypeError: If a node is not an
              instance of :class:`binarytree.Node`.
          :raise binarytree.exceptions.NodeValueError: If a node value is not a
              number (e.g. int, float).
  
          **Example**:
  
          .. doctest::
  
              >>> from binarytree import Node
              >>>
              >>> root = Node(1)
              >>> root.left = Node(2)
              >>> root.right = root  # Cyclic reference to root
              >>>
              >>> root.validate()  # doctest: +IGNORE_EXCEPTION_DETAIL
              Traceback (most recent call last):
               ...
              NodeReferenceError: cyclic node reference at index 0