The new length (or number of rows for a data frame)
sep
When not specifying compressFun and compressFunPart, the
paste function is used to combine elements, and in that case, sep is
passed to paste as separator.
compressFun
If specified, when compressing streams, instead of pasting
elements together using separator sep, the vectors are passed to function
compressFun, which must accept a vector (to compress) and a single integer
(with the desired resulting length of the vector).
compressFunPart
A function to apply to the segments that are
automatically created; this can be passed instead of compressFun.