Changes in version 0.0.5
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* change interval options to `__generatedNamespace__`

Changes in version 0.0.4
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* completely replace `is` with `inherits`

Changes in version 0.0.3
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* remove dependency of `methods` package
* `value_fun` is deleted after setting every option
* function is attached to the value if option values is set as a dynamic function
  so that opt(opt(READ.ONLY = FALSE)) will be correct if there are option values 
  set as dynamic functions.

Changes in version 0.0.2
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* use knitr to build the vignette.
* add .private and .visible fields for making options specification. Private options
  can only be modified if the namespace in which the options function is generated and the
  namespace where the options function is called is the same namespace. Invisible options
  will not be returned when getting the option values.

Changes in version 0.0.1
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* the first release
