old_contents: class Node(object):
      def __init__(self, inbound_nodes=[]):
          # 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
  
  class Input(Node):
      def __init__(self):
          # An Input node has no inbound nodes, so no need to pass anything to the
          # Node instantiator.
          Node.__init__(self)
  
      # NOTE: Input node is the only node where the value may be passed as an
      # argument to forward().
      #
      # All other node implementations should get the value of the previous node
      # from self.inbound_nodes
      #
      # Example:
      # val0 = self.inbound_nodes[0].value
      def forward(self, value=None):
          # Overwrite the value if one is passed in.
          if value is not None:
              self.value = value
  
  class Add(Node):
      def __init__(self, x, y):
          # Add class takes 2 inbound nodes, x and y, and adds the values of those
          # nodes.
          Node.__init__(self, [x, y])
  
      def forward(self):
          """
          Set the value of this node (`self.value`) to the sum of it's inbound_nodes.
  
          Your code here!
          """
          self.value = 0
          for n in self.inbound_nodes:
              self.value += n.value
  
new_contents: class Node(object):
      def __init__(self, inbound_nodes=[]):
          # 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
  
  class Input(Node):
      def __init__(self):
          # An Input node has no inbound nodes, so no need to pass anything to the
          # Node instantiator.
          Node.__init__(self)
  
      # NOTE: Input node is the only node where the value may be passed as an
      # argument to forward().
      #
      # All other node implementations should get the value of the previous node
      # from self.inbound_nodes
      #
      # Example:
      # val0 = self.inbound_nodes[0].value
      def forward(self, value=None):
          # Overwrite the value if one is passed in.
          if value is not None:
              self.value = value
  
  class Add(Node):
      def __init__(self, *inputs):
          # Add class takes 2 inbound nodes, x and y, and adds the values of those
          # nodes.
          Node.__init__(self, inputs)
  
      def forward(self):
          """
          Set the value of this node (`self.value`) to the sum of it's inbound_nodes.
  
          Your code here!
          """
          self.value = 0
          for n in self.inbound_nodes:
              self.value += n.value
  
subject: Modify __init__ of Add node