public GroupingOperation putAlias(String id, GroupingExpression exp) {
        aliases.put(id, exp);
        return this;
    }
Registers an alias with this operation. An alias is made available to expressions in both this node and all child
nodes.

@param id the id of the alias to put
@param exp the expression to associate with the id
@return this, to allow chaining
container-search/src/main/java/com/yahoo/search/grouping/request/GroupingOperation.java