NEW FEATURES
o setkey no longer copies the whole table and should be
faster for large tables. Each column is reordered by reference
(in C) using one column of working memory, FR#1006. User
defined attributes on the original table are now also
retained (thanks to Thell Fowler for reporting).
o A new symbol .N is now available to j, containing the
number of rows in the group. This may be useful when
the column names are not known in advance, for
convenience generally, and for efficiency.
(The .N feature is needed by package randomNames, now that group
variables are length 1)
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