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Summary
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Summary
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* Hierarchical library units address important issues

   - Direct support for subsystems
   - Extension without recompilation
   - Separation of concerns with controlled sharing of visibility

* Parents should document assumptions for children

   - "These must always be in ascending order!"

* Children cannot misbehave unless imported ("with'ed")

* Not uncommon for two package specs to be interdependent

  * :ada:`limited with` can resolve circularity
  * May involve rethinking your type definitions
