R6 2.0.1
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* Superclasses are checked only on object instantiation, not on class creation.

* Added documentation about debugging and finalizers.

R6 2.0
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* [BREAKING CHANGE] Added `portable` option, which allows inheritance across
  different package namespaces, and made it the default.

* Added `set()` method on class generator object, so new fields and methods can
  be added after the generator has been created.

* All of the functions involved in instantiating objects are encapsulated in
  an environment separate from the R6 namespace. This means that if a
  generator is created with one version of R6, saved, then restored in a new R
  session that has a different version of R6, there shouldn't be any problems
  with compatibility.

* Methods are locked so that they can't be changed. (Fixes #19)

* Inheritance of superclasses is dynamic; instead of reading in the superclass
  when a class is created, this happens each time an object is instantiated.
  (Fixes #12)

* Added trailing newline when printing R6 objects. (Thanks to Gabor Csardi)

* The `print` method of R6 objects can be redefined. (Thanks to Gabor Csardi)

R6 1.0.1
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* First release on CRAN.

* Removed pryr from suggested packages.

R6 1.0
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* First release