code: public void do_not_split( ) {
        if( _pid == NO_PARENT) return; // skip root
        DecidedNode dn = _tree.decided(_pid);
        for( int i=0; i<dn._nids.length; i++ )
          if( dn._nids[i]==_nid )
            { dn._nids[i] = ScoreBuildHistogram.UNDECIDED_CHILD_NODE_ID; return; }
        throw H2O.fail();
      }
docs: perfect prediction here, and we want to turn into a leaf.