public ConfigPayloadBuilder set(int index) {
            verifyIndex();
            ConfigPayloadBuilder p = new ConfigPayloadBuilder(getInnerArrayDef(name));
            elements.put(index, p);
            return p;
        }
Set Create a payload object for the given index and return it. Any previously stored version will be
overwritten.

@param index Index of new element.
@return The payload builder for the newly created slime object.
config/src/main/java/com/yahoo/vespa/config/ConfigPayloadBuilder.java