old_contents: class Node(object):
    def __init__(self):
      # Node(s) from which this Node receives values
      self.inbound_nodes = inbound_nodes
      # Node(s) to which this Node passes values
      self.outbound_nodes = []
      # For each inbound Node here, add this Node as an outbound to that Node.
      for n in self.inbound_nodes:
        n.outbound_nodes.append(self)
  
      # A calculated value
      self.value = None
  
new_contents: class Node(object):
    def __init__(self):
      # Node(s) from which this Node receives values
      self.inbound_nodes = inbound_nodes
      # Node(s) to which this Node passes values
      self.outbound_nodes = []
      # For each inbound Node here, add this Node as an outbound to that Node.
      for n in self.inbound_nodes:
        n.outbound_nodes.append(self)
  
      # A calculated value
      self.value = None
  
    def forward(self):
      """
      Forward propagation.
  
      Compute the output value based on `inbound_nodes` and store the result in
      self.value.
      """
      raise NotImplemented
  
subject: Add forward function declaration to Class Node