}

    def "can realize a filtered set of providers and order is correct"() {
        when:
        source.addPending(provider("foo"))
        source.addPending(provider(new StringBuffer("bar")))
        source.addPending(provider(new StringBuffer("baz")))
        source.addPending(provider("fizz"))
        source.addPendingCollection(setProvider(new StringBuffer("fuzz"), new StringBuffer("bazz")))

        then:

        when:
        source.addPending(provider1)
        source.addPending(provider2)
        source.addPending(provider3)
        source.realizePending(SomeType.class)

        then:
        source.iteratorNoFlush().collect() == ["provider1", "provider3"]
    }

    def "can remove pending elements"() {
        when:
        source.addPending(provider1)
        source.addPending(provider2)

        when:
        source.addPending(provider)
        source.addPending(provider)

        then:
        source.size() == 2
        source.iterator().collect() == ["foo", "foo"]
    }

    def "can add an element as both a provider and a realized value"() {
        when:
        source.add("foo")
        source.addPending(provider("foo"))

        then:

    }

    def "can add only providers"() {
        when:
        source.addPending(provider("foo"))
        source.addPending(provider("bar"))
        source.addPending(provider("baz"))
        source.addPending(provider("fizz"))

        then:
        source.iteratorNoFlush().collect() == []

        and:

        when:
        def provider1 = Providers.of("v1").withSideEffect(sideEffect1)
        source.addPending(provider1)
        def provider2 = Providers.of("v2").withSideEffect(sideEffect2)
        source.addPending(provider2)
        def provider3 = Providers.of("v3").withSideEffect(sideEffect3)
        source.addPending(provider3)
        def provider4 = propertyFactory.listProperty(String).with {

        def provider = provider("foo")

        when:
        source.addPending(provider)
        source.addPending(provider)

        then:
        source.size() == 1
        source.contains("foo")
    }

    def "an element added as both a provider and a realized value is not duplicated"() {
        when:
        source.add("foo")
        source.addPending(provider("foo"))

        then:

    }

    @Override
    public void realizePending(Class<?> type) {
        collection.realizePending(type);
    }

    @Override
    public boolean addPending(ProviderInternal<? extends S> provider) {
        return collection.addPending(provider);
    }

    @Override
    public boolean removePending(ProviderInternal<? extends S> provider) {
        return collection.removePending(provider);
    }

import org.gradle.api.internal.provider.ProviderInternal;

public interface PendingSource<T> {
    void realizePending();

    void realizePending(Class<?> type);

    boolean addPending(ProviderInternal<? extends T> provider);

    boolean removePending(ProviderInternal<? extends T> provider);

    boolean addPendingCollection(CollectionProviderInternal<T, ? extends Iterable<T>> provider);

                            seen = true;
                        }
                    }
                }
            }
        }
    }

    @Override
    public boolean addPending(ProviderInternal<? extends T> provider) {
        modCount++;
        Element<T> element = cachingElement(provider);
        if (!getInserted().contains(element)) {
            addPendingElement(element);

        modCount++;
        return getInserted().add(new Element<T>(element));
    }

    @Override
    public boolean addRealized(T value) {
        return true;
    }

    @Override
    public boolean addPending(ProviderInternal<? extends T> provider) {
        modCount++;
        return addPendingElement(cachingElement(provider));
    }

    @Override