A Rarefac object is a small (S3) extension to a matrix. The first column contains
the cumulative number of unique gene clusters found when considering 1,2,...,G genomes in a pan-matrix.
Thus, the Rarefac object is a matrix with G rows. Any additional columns will hold similar
numbers, but for random shufflings of the genome's ordering. A Rarefac object is typically
created by the function rarefaction.
The plot.Rarefac function will display the content of the Rarefac object as a plot
of the mean value in rows 1,2,...,G, where G is the total number of genomes in the study.
The summary.Rarefac function will display a text giving the same information as
plot.Rarefac.