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A general description is here.
It seems file.access() can be unreliable both ways: that is it can also report that access is barred when it is not. (Seen on network file systems.) So install.packages() to a single library no longer makes use of file.access() but checks directory creation instead: it is still possible that you can create a directory but not be allowed to create a .dll or .chm file, at least under Vista.
When R_COMPLETION is set to FALSE, TAB is no longer ignored, but advances to the next tab stop in Rterm (the pre-2.5.0 behaviour) and inserts a space in Rgui.
The canvas used when plotting on a jpeg() or bmp() device with a transparent background (most often by copying from a screen device with the default transparent background) is now white and not grey.
o It seems file.access() can be unreliable both ways: that is it can also report that access is barred when it is not. (Seen on network file systems.) So install.packages() to a single library no longer makes use of file.access() but checks directory creation instead: it is still possible that you can create a directory but not be allowed to create a .dll or .chm file, at least under Vista.