code: def add_bits4subtree_ids(self, relevant_ids):
          
          if relevant_ids:
              checking = True
          else:
              checking = False
              relevant_ids = {}
              bit = 1
          self.bits2internal_node = {}
          for node in self.postorder_node_iter():
              p = node._parent
              if p is None:
                  if not node.is_leaf:
                      self.bits2internal_node[node.bits4subtree_ids] = node
                  continue
              if not hasattr(p, 'bits4subtree_ids'):
                  p.bits4subtree_ids = 0
              i = node._id
              # _LOG.debug('node._id ={}'.format(i))
              # _LOG.debug('Before par mrca... = {}'.format(p.bits4subtree_ids))
              if checking:
                  b = relevant_ids.get(i)
                  if b:
                      if node.is_leaf:
                          node.bits4subtree_ids = b
                      else:
                          node.bits4subtree_ids |= b
              else:
                  if node.is_leaf:
                      relevant_ids[i] = bit
                      node.bits4subtree_ids = bit
                      bit <<= 1
              if not node.is_leaf:
                  self.bits2internal_node[node.bits4subtree_ids] = node
              # _LOG.debug('while add bitrep... self.bits2internal_node = {}'.format(self.bits2internal_node))
              p.bits4subtree_ids |= node.bits4subtree_ids
          return relevant_ids
docs: Adds a long integer bits4subtree_ids to each node (Fails cryptically if that field is already present!)
          relevant_ids can be a dict of _id to bit representation.
          If it is not supplied, a dict will be created by registering the leaf._id into a dict (and returning the dict)
          the bits4subtree_ids will have a 1 bit if the _id is at or descended from this node and 0 if it is not
              in this subtree.
          Returns the dict of ids -> longs
          Also creates a dict of long -> node mappings for all internal nodes. Stores this in self as bits2internal_node