@Override
        public void add(E input) {
            if (groupCollector == null) {
                groupCollector = groupExpression.createGroupCollector();
            }
            groupCollector.add(input);
        }
{@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