Class "packedMatrix" is the virtual class of dense
symmetric or triangular matrices in "packed" format, storing only
the choose(n+1,2) == n*(n+1)/2 elements of the upper or
lower triangle of an n-by-n matrix. It is used to
define common methods for efficient subsetting, transposing, etc.
of its proper subclasses: currently "[dln]spMatrix"
(packed symmetric), "[dln]tpMatrix" (packed triangular),
and subclasses of these, such as
"dppMatrix".