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A general description is here.
The directory pointed to by ‘USER_LOCAL’ can now have architecture-specific sub-directories ‘lib/i386’ and ‘lib/x64’.
‘tools::dependsOnPkg()’ now accepts the shorthand ‘dependencies = "all"’.
‘parallel::clusterExport()’ now allows specification of an environment from which to export.
When checking examples under more than one sub-architecture, ‘R CMD check’ now uses a separate directory ‘examples_arch’ for each sub-architecture, and leaves the output in file ‘pkgname-Ex_arch.Rout’. Some packages expect their examples to be run in a clean directory ....