return delegate.concat(fallback, primary);
    }

    @Override
    public ImmutableAttributes safeConcat(ImmutableAttributes attributes1, ImmutableAttributes attributes2) throws AttributeMergingException {
        return delegate.safeConcat(attributes1, attributes2);
    }

    @Override
    public ImmutableAttributes compileScope(ImmutableAttributes original) {

        given:
        def set1 = factory.concat(factory.of(FOO, "foo1"), factory.of(BAR, "bar1"))
        def set2 = factory.concat(factory.of(FOO, "foo1"), factory.of(BAR, "bar2"))

        when:
        factory.safeConcat(set1, set2)

        then:
        AttributeMergingException e = thrown()
        e.attribute == BAR
        e.leftValue == "bar1"
        e.rightValue == "bar2"
    }

     *
     * Attributes with same name but different type are considered equal for the purpose of merging.
     */
    ImmutableAttributes safeConcat(ImmutableAttributes attributes1, ImmutableAttributes attributes2) throws AttributeMergingException;

        ImmutableAttributes attributes = ((AttributeContainerInternal) append).asImmutable();
        if (mergedConstraintAttributes.isEmpty()) {
            return attributes;
        }
        return attributesFactory.safeConcat(mergedConstraintAttributes.asImmutable(), attributes);
    }

    private ImmutableAttributes appendAttributes(ImmutableAttributes dependencyAttributes, SelectorState selectorState) {
        try {

                }
            }
        }
        return current;
    }

    @Override
    public ImmutableAttributes safeConcat(ImmutableAttributes attributes1, ImmutableAttributes attributes2) throws AttributeMergingException {
        if (attributes1 == ImmutableAttributes.EMPTY) {
            return attributes2;
        }