Defines a list object composed of `formula`

s. 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`

.

## Arguments

- ...
`formula`

s, possibly named

## Value

A `list`

of class `formula_list`

.

## Details

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.