input: Write a function to compute the sum of all elements in a binary tree. The function should take the root node of the binary tree as an input and return the sum of all its elements. If the tree is empty (the root node is None), the function should return 0.
  
  assert sum_tree(None)==0
  assert sum_tree(Node(1, Node(2, Node(4), Node(5)), Node(3)))==15
  assert sum_tree(Node(10, Node(20), Node(30)))==60
  assert sum_tree(Node(100, Node(200, Node(300)), Node(400)))==1000
  assert sum_tree(Node(5, None, Node(10, Node(15))))==30
output: def sum_tree(root):
      if root is None:
          return 0
      return root.value + sum_tree(root.left) + sum_tree(root.right)