code: def encode_branch_node(left_child_node_hash, right_child_node_hash):
      
      validate_is_bytes(left_child_node_hash)
      validate_length(left_child_node_hash, 32)
      validate_is_bytes(right_child_node_hash)
      validate_length(right_child_node_hash, 32)
      return BRANCH_TYPE_PREFIX + left_child_node_hash + right_child_node_hash
docs: Serializes a branch node