old_contents: from .models import Node, Info, Transmission
  from sqlalchemy import ForeignKey, Column, String
  import random
  
  
  class Source(Node):
      __tablename__ = "source"
      __mapper_args__ = {"polymorphic_identity": "generic_source"}
  
      uuid = Column(String(32), ForeignKey("node.uuid"), primary_key=True)
  
      def _selector(self):
          return []
  
      def create_information(self):
          """Generate new information."""
          return NotImplementedError
  
  
  class RandomBinaryStringSource(Source):
      """An agent whose genome and memome are random binary strings. The source
      only transmits; it does not update.
      """
  
      __mapper_args__ = {"polymorphic_identity": "random_binary_string_source"}
  
      @staticmethod
      def _data():
          return "".join([str(random.randint(0, 1)) for i in range(2)])
  
new_contents: from .models import Node, Info, Transmission
  from sqlalchemy import ForeignKey, Column, String
  import random
  
  
  class Source(Node):
      __tablename__ = "source"
      __mapper_args__ = {"polymorphic_identity": "generic_source"}
  
      uuid = Column(String(32), ForeignKey("node.uuid"), primary_key=True)
  
      def _selector(self):
          return None
  
      def create_information(self):
          """Generate new information."""
          return NotImplementedError
  
  
  class RandomBinaryStringSource(Source):
      """An agent whose genome and memome are random binary strings. The source
      only transmits; it does not update.
      """
  
      __mapper_args__ = {"polymorphic_identity": "random_binary_string_source"}
  
      @staticmethod
      def _data():
          return "".join([str(random.randint(0, 1)) for i in range(2)])
  
subject: Make the default selector null