For matrices, 2-dim tables and data frames, Some() returns
some n rows when n > 0 or all but the
some n rows when n < 0. Some.matrix() is not exported (unlike head.matrix).
If a matrix has no row names, then Some() will add row names of
the form "[n,]" to the result, so that it looks similar to the
last lines of x when printed. Setting addrownums =
FALSE suppresses this behaviour.
I desisted from implementing interfaces for tables, ftables and functions, as this would not make much sense.