Every modeling function in S-PLUS needs a model formula, that is,
a predictor or predictors and a response. The format for such a
formula is the response, a ~ (tilde) meaning ``modeled by'', and
the predictors. The predictors should be separated by +, or
* if an interaction between two predictors is being modeled.
An intercept term is included with the predictors by default, to remove
it use -1 as a predictor.