// approximation.
		i := 1
		if last := len(sample.Location) - 1; last < i {
			i = last
		}
		for ; i >= 0; i-- {
			l := sample.Location[i]
			locNodes := locationMap.get(l.ID)
			for ni := len(locNodes) - 1; ni >= 0; ni-- {
				n := locNodes[ni]
				if n == nil {
					residual = true
					continue
				}
				// Add cum weight to all nodes in stack, avoiding double counting.
				_, sawNode := seenNode[n]

		labels := joinLabels(sample)
		// Group the sample frames, based on a global map.
		for i := len(sample.Location) - 1; i >= 0; i-- {
			l := sample.Location[i]
			locNodes := locationMap[l.ID]
			for ni := len(locNodes) - 1; ni >= 0; ni-- {
				n := locNodes[ni]
				if n == nil {
					residual = true
					continue
				}
				// Add cum weight to all nodes in stack, avoiding double counting.