default <V> V computeIfAbsent(
            final @NonNull CloudKeyHolder<V> keyHolder,
            final @NonNull Function<@NonNull CloudKey<V>, V> defaultFunction
    ) {
        return this.computeIfAbsent(keyHolder.key(), defaultFunction);
    }
Returns the value associated with the given {@code keyHolder} if it exists, else computes and stores the
value returned by the given {@code defaultFunction} and then returns it.

@param <V>             the type of the value
@param keyHolder       the holder of the key
@param defaultFunction the function used to generate the value in case it's missing
@return the present or computed value
cloud-core/src/main/java/org/incendo/cloud/key/MutableCloudKeyContainer.java