This is a generic function, with methods supplied for matrices, data
frames and vectors (including lists). Packages and users can add
further methods.

For ordinary vectors, the result is simply
`x[subset & !is.na(subset)]`

.

For data frames, the `subset`

argument works on the rows. Note
that `subset`

will be evaluated in the data frame, so columns can
be referred to (by name) as variables in the expression (see the examples).

The `select`

argument exists only for the methods for data frames
and matrices. It works by first replacing column names in the
selection expression with the corresponding column numbers in the data
frame and then using the resulting integer vector to index the
columns. This allows the use of the standard indexing conventions so
that for example ranges of columns can be specified easily, or single
columns can be dropped (see the examples).

The `drop`

argument is passed on to the indexing method for
matrices and data frames: note that the default for matrices is
different from that for indexing.

Factors may have empty levels after subsetting; unused levels are
not automatically removed. See `droplevels`

for a way to
drop all unused levels from a data frame.