@Override
    public GroupCollector<E, R> createGroupCollector() {
        return new GroupCollectorImpl();
    }
{@code MixinGroupExpression} provides nesting support for {@link GroupExpression} instances

@param <E>
@param <F>
@param <R>
querydsl-core/src/main/java/com/querydsl/core/group/MixinGroupExpression.java