R6 2.1.1
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* Fixed a bug with printing R6 objects when a `[[` method is defined for the
  class. (#70)

* Fixed cloning of objects that call a `super` method which accesses `private`.
  (#72)

R6 2.1.0
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* Added support for making clones of R6 objects with a `clone()` method on R6
  objects. The `deep=TRUE` option allows for making clones that have copies of
  fields with reference semantics (like other R6 objects). (#27)

* Allow adding public or private members when there were no public or private
  members to begin with. (#51)

* Previously, when an R6 object was printed, it accessed (and called) active
  bindings. Now it simply reports that a field is an active binding. (#37, #38.
  Thanks to Oscar de Lama)

* Printing private members now works correctly for portable R6 objects. (#26)

* The 'lock' argument has been renamed to 'lock_objects'. Also, there is a new
  argument, 'lock_class', which can prevent changes to the class. (#52)

* Fixed printing of NULL fields.

R6 2.0.1
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* A superclass is validated on object instantation, not on class creation.

* Added `debug` and `undebug` methods to generator object.

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
