public Hop setDirective(int i, HopDirective directive) {
        selector.set(i, directive);
        return this;
    }
Sets the directive at a given index.

@param i         The index at which to set the directive.
@param directive The directive to set.
@return This, to allow chaining.
messagebus/src/main/java/com/yahoo/messagebus/routing/Hop.java