split(x, f, drop = FALSE):
Splits x into a CompressedSplitDataFrameList,
according to f, dropping elements corresponding to
unrepresented levels if drop is TRUE.
mstack(..., .index.var = "name"): Stacks the data frames
passed as through ..., using .index.var as the index
column name. See stack.
See Also
DataTable,
Vector, and
RangedData, which makes heavy use of this class.