# Introduction to Aggregates

- [Introduction](introduction.md)

- [Restrictions on Sets and Sequences](restriction.md)

  - [Universe of a Set or Sequence](restriction.md#universe-of-a-set-or-sequence)

  - [Modifying the Universe of a Set or Sequence](restriction.md#modifying-the-universe-of-a-set-or-sequence)

  - [Parents of Sets and Sequences](restriction.md#parents-of-sets-and-sequences)

- [Dual iterators in constructors](dual-iterators.md)

- [Nested Aggregates](nesting.md)

  - [Multi-indexing](nesting.md#multi-indexing)