Split formula into individual terms

## See also

Functions used to help convert formulas into model designs
`as_string_formula()`

,
`construct_re()`

,
`construct_rw()`

,
`enw_formula()`

,
`enw_manual_formula()`

,
`parse_formula()`

,
`re()`

,
`remove_rw_terms()`

,
`rw()`

,
`rw_terms()`