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Description
Currently the section on variance says:
A longer introduction to these concepts can be found on Wikipedia and in PEP 483; here we just show how to control a type checker’s behavior.
Now that we have a core concepts section in the spec, it would be good for the spec to also derive the rationale for variance from those core concepts, rather than deferring to the (ten-year-old and not updatable) PEP 483.
We should also add glossary entries for variance terminology: invariant, covariant, contravariant, and variance.