public void select(RoutingContext ctx) {
        Route route = new Route(ctx.getRoute());
        route.setHop(0, getRecipient(ctx));
        ctx.addChild(route);
    }
Creates an instance of a subset service policy. The parameter string is parsed as an integer number that is the
number of services to include in the set to choose from.

@param param The number of services to include in the set.
documentapi/src/main/java/com/yahoo/documentapi/messagebus/protocol/SubsetServicePolicy.java