This function extracts random walk terms
denoted using `rw()`

from a formula so that they can be
processed on their own.

## Value

A character vector containing the random walk terms that have been identified in the supplied formula.

## Reference

This function was adapted from code written
by J Scott (under an MIT license) as part of
the `epidemia`

package (https://github.com/ImperialCollegeLondon/epidemia/).

## 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()`

,
`remove_rw_terms()`

,
`re()`

,
`rw()`

,
`split_formula_to_terms()`

## Examples

```
epinowcast:::rw_terms(~ 1 + age_group + location)
#> character(0)
epinowcast:::rw_terms(~ 1 + age_group + location + rw(week, location))
#> [1] "rw(week, location)"
```