Given a data frame and the names of variable, doubles the
data frame for each variable with a new category
"All" (or optionally "Combined").
A new variable .marginal. is added to the resulting data frame,
with value "" if the observation is an original one, and with
value equal to the names of the variable being marginalized (separated
by commas) otherwise.
location for marginal category within factor variable
specifying categories. Set to "first" to override the
default - to put a category with value label as the first category.