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A general description is here.
‘is.R()’ is deprecated as no other S dialect is known to be in use (and this could only identify historical dialects, not future ones).
For now, further information on calls can be obtained by setting environment variable ‘_R_DEPRECATED_IS_R_’. Value ‘warn’ gives a warning as per the default except immediately, value ‘error’ turns this warning into an error (so by default gives a traceback), value ‘traceback’ gives a tracebeck in a message (not a warning, as that may truncate) and value ‘"where"’ gives the call to the function (if any) calling ‘is.R’ and which namespace it is from.