`identify_missing`

is a function used to activate missing data
weighting and potentially supply a formula and estimation function.

`identify_missing(formula = ~1, estimator = estimate_missing_glmnet)`

## Arguments

- formula
A formula object that can be used to build the missingness
model. Note this does not need to include treatment since the missingness
models are fit within treatment groups. Defaults to a null model, ~1.

- estimator
An optional argument providing an estimator function for the missingness model.

## Value

An object that can be used by specify_models to add information about
missingness to the data description function.

## Author

M. Alan Brookhart and Alexander Breskin