The function specifies the model formula for fitting `lm_mverse()` and
`glm_mverse()`. You can list the model specification formulae individually
or use `covariates`

option paired with one or more formulae.

`formula_branch(..., covariates = NULL, name = NULL)`

- ...
branch definition expressions.

- covariates
(optional) A character vector of optional covariates. Each unique combination of the supplied covariates is translated into a unique branch option. See Details.

- name
(optional) Name for the new formula.

a `formula_branch`

object.

The optional argument `covariates`

is allows you to specify a set of
optional covariates in addition to other independent variable such as
treatment variables and blocking variables which are specified using formula.
For each covariate provided, a branch is added to the multiverse with the
option to include or exclude the covariate in the model.

For example, `formula_branch(y ~ x, covariates = c("c1", "c2"))`

creates
the following 4 model specifications:

`y ~ x`

`y ~ x + c1`

`y ~ x + c2`

`y ~ x + c1 + c2`

Here, `y`

is the outcome variable and `x`

may be a treatment
variable in an experiment setting. `c1`

and `c2`

may be additional
covariates about the experiment units that may or may not be relevant.

Other formula branch functions:
`add_formula_branch()`

```
# Define a formula branch.
model_specifications <- formula_branch(
y ~ femininity,
y ~ femininity + hurricane_strength,
y ~ femininity * hurricane_strength
)
# Create a mverse, add the branch.
mv <- create_multiverse(hurricane) %>%
add_formula_branch(model_specifications)
# Specify the covariates separately.
model_specifications <- formula_branch(
y ~ femininity,
covariates = c("hurricane_strength", "Year", "Category", "NDAM")
)
model_specifications
#> <unnamed branch> Options
#> - (y ~ femininity)
#> Covariates
#> - hurricane_strength
#> - Year
#> - Category
#> - NDAM
```