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Understanding the Traits Component
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.. note::

   TODO\: an actual tutorial, for now the main points are listed.

- Types are often viral because interactions between classes happen through the
  types known to those classes, e.g., for an object of type ``T`` to interact
  with an object of type ``U``, ``T`` must be able to understand the types
  ``U`` uses to access state and vice versa.
- With many Chemist objects actually consisting of a number of different
  classes, we need to synchronize types across those objects.
- Also used for the template meta-programming to ensure const-ness is correct.
- Traits allows a single source of truth for types associated a class.