Supplier<T> s = asSupplier(supplier);
        if (cache == null) {
            return s.get();
        } else {
            return cache.computeIfAbsent(groupId, artifactId, version, tag, s);
        }
    }

    private static <T> T cache(ModelCache cache, Source source, String tag, Callable<T> supplier) {
        Supplier<T> s = asSupplier(supplier);

        protected Activation.Builder transformActivation_ActiveByDefault(
                Supplier<? extends Activation.Builder> creator, Activation.Builder builder, Activation target) {
            return builder;
        }

        @Override
        protected Activation.Builder transformActivation_File(
                Supplier<? extends Activation.Builder> creator, Activation.Builder builder, Activation target) {

        protected Activation.Builder transformActivation_ActiveByDefault(
                Supplier<? extends Activation.Builder> creator, Activation.Builder builder, Activation target) {
            return builder;
        }

        @Override
        protected Activation.Builder transformActivation_File(
                Supplier<? extends Activation.Builder> creator, Activation.Builder builder, Activation target) {

            Callable<T> supplier) {
        Supplier<T> s = asSupplier(supplier);
        if (cache == null) {
            return s.get();
        } else {
            return cache.computeIfAbsent(groupId, artifactId, version, tag.getName(), s);
        }
    }

    private static <T> T cache(ModelCache cache, Source source, ModelCacheTag<T> tag, Callable<T> supplier) {

        throw (T) t; // rely on vacuous cast
    }

    static class LazyMap<K, V> extends AbstractMap<K, V> {
        private final Supplier<Map<K, V>> supplier;
        private volatile Map<K, V> delegate;

        LazyMap(Supplier<Map<K, V>> supplier) {
            this.supplier = supplier;
        }

        @Override
        public Set<Entry<K, V>> entrySet() {
            if (delegate == null) {

        // is always available in the core and likely always will be, but we may have another ConfigurationProcessor
        // present supplied by the user. The rule is that we only allow the execution of one ConfigurationProcessor.
        // If there is more than one then we execute the one supplied by the user, otherwise we execute the
        // default SettingsXmlConfigurationProcessor.
        //

        return buffer.toString();
    }

    /**
     * Sets the value of the context value of this project identified by the given key. If the supplied value is
     * <code>null</code>, the context value is removed from this project. Context values are intended to allow core
     * extensions to associate derived state with project instances.
     */