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A general description is here.
When a character value is used for the ‘EXPR’ argument in ‘switch()’, a warning is given if more than one unnamed alternative value is given. This will become an error in R 2.13.0.
New reference class utility methods ‘copy’, ‘field’, ‘getRefClass’ and ‘getClass’ have been added.
The parser for ‘NAMESPACE’ files ignored misspelled directives, rather than signalling an error. For 2.12.x a warning will be issued, but this will be correctly reported as an error in later releases. (Reported by Charles Berry.)