code: function model(M, prop) {
    const bmiLens = L(
      prop,
      // if we don't have <prop> property yet, then use these default values
      L.defaults({weight: 80, height: 180}),
      // add "read-only" bmi property to our BMI model that is derived from weight and height
      L.augment({bmi: ({weight: w, height: h}) => Math.round(w / (h * h * 0.0001))})
    )
    return M.lens(bmiLens)
  }
docs: Let's define the model function so that it takes the parent state and property containing the BMI counter state. We are making this BMI model as a separate function so that we could re-use it if needed