variable specification such as would be created by a dialog.
subset
subset expression for rows.
na.rm
logical indicating whether to remove rows with missing values.
VALUE:
a data.frame produced by model.frame.
DETAILS:
This function is used to create a data.frame containing specified
variables, taking into account a subset expression and whether to
omit missings. It is useful when passing data to a function which
can handle a data.frame, but which doesn't accept the other arguments
present here.