@Override
        public R get() {
            if (groupCollector != null) {
                F output = groupCollector.get();
                mixinGroupCollector.add(output);
                groupCollector = null;
            }
            return mixinGroupCollector.get();
        }
{@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