private static void selectionToString(StringBuilder sb, List<Selection> nodes) {
        boolean first = true;
        for (Selection p : nodes) {
            if (first) {
                first = false;
            } else {
                sb.append(',');
            }
            sb.append(p.toString());
        }
    }
Parses a search path and select nodes from the given cluster based on it.

@param searchPath unparsed search path expression (see: model.searchPath in Search API reference)
@param cluster the search cluster from which nodes are selected
@throws InvalidSearchPathException if the searchPath is malformed
@return list of nodes chosen with the search path, or an empty list in which
        case some other node selection logic should be used
container-search/src/main/java/com/yahoo/search/dispatch/SearchPath.java