code: public Selector intern(InternTable table)
    {
      // This implementation is inherited by Identifier and Literal which have no children,
      // but is overridden in Operator, which does.
      Selector me = (Selector) table.get(this);
      if (me != null)
      {
        me.incRefCount();
        return me;
      }
      else
      {
        uniqueId = table.getNextUniqueId();
        refCount++;
        table.put(this, this);
        return this;
      }
    }
docs: Replaces this Selector tree with an equal() Selector tree that is "uniquely
  identified".
  
  This means that (1) all of its subtrees are uniquely identified
  (recursive definition), (2) it has a uniqueId assignment, (3) relative to the
  supplied InternTable, equal subtrees always have the same uniqueIds, and (4) relative
  to the supplied InternTable, unequal subtrees always have different uniqueIds.
  
  @param table the InternTable that will decide uniqueness, assign uniqueIds and store
  uniquely identified subtrees.
  
  @return a uniquely identified tree equal() to the this tree