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A general description is here.
Dependence specifications for R or packages in the Depends field in a DESCRIPTION file can now make use of operators < > == and != (in addition to <= and >=): such packages will not be installable nor loadable in R < 2.7.0.
There can be multiple mentions of R or a package in the Depends field in a DESCRIPTION file: only the first mention will be used in R < 2.7.0.
When plot.default() is called with y=NULL it now calls Axis() with the 'y' it constructs rather than use the default axis.
dbeta(.., ncp) is now more accurate for large ncp.