public GroupingExpression getAlias(String id) {
        if (aliases.containsKey(id)) {
            return aliases.get(id);
        } else if (parent != null) {
            return parent.getAlias(id);
        } else {
            return null;
        }
    }
Returns the alias associated with the given name. If no alias can be found in this node, this method queries its
parent grouping node. If the alias still can not be found, this method returns null.

@param id the id of the alias to return
@return the expression associated with the id
container-search/src/main/java/com/yahoo/search/grouping/request/GroupingOperation.java