docs: def sysmeta_add_preferred(sysmeta_pyxb, node_urn):
      """Add a remote Member Node to the list of preferred replication targets to this
      System Metadata object.
  
      Also remove the target MN from the list of blocked Member Nodes if present.
  
      If the target MN is already in the preferred list and not in the blocked list, this
      function is a no-op.
  
      Args:
        sysmeta_pyxb : SystemMetadata PyXB object.
          System Metadata in which to add the preferred replication target.
  
          If the System Metadata does not already have a Replication Policy, a default
          replication policy which enables replication is added and populated with the
          preferred replication target.
  
        node_urn : str
          Node URN of the remote MN that will be added. On the form
         ``urn:node:MyMemberNode``.
  
      """
      if not has_replication_policy(sysmeta_pyxb):
          sysmeta_set_default_rp(sysmeta_pyxb)
      rp_pyxb = sysmeta_pyxb.replicationPolicy
      _add_node(rp_pyxb, 'pref', node_urn)
      _remove_node(rp_pyxb, 'block', node_urn)