list composed of formulasDefines a list object composed of formulas. The purpose of this
class of object is to impose structure of the list to ensure that it
can be used to obtain RHS-LHS pairs and will be able to be
parsed in domir.
formula_list(...)A list of class formula_list.
formulas, possibly named
The formula_list requires that each element of the list is a formula
and that each formula is unique with a different, non-NULL
dependent variable/response.