(R-admin.info)Add-on packages

It is helpful to use the correct terminology.  A _package_ is loaded
from a _library_ by the function 'library()'.  Thus a library is a
directory containing installed packages; the main library is
'R_HOME/library', but others can be used, for example by setting the
environment variable 'R_LIBS' or using the R function '.libPaths()'.  To
avoid any confusion you will often see a library directory referred to
as a 'library tree'.