validator(value) {
        const acceptableValues = [ ALL_CHILDREN, ALL_DESCENDANTS, LEAF_CHILDREN, LEAF_DESCENDANTS ]
        return includes(acceptableValues, value)
      }
Used in conjunction with `showCount` to specify which type of count number should be displayed.
Acceptable values:
  - "ALL_CHILDREN"
  - "ALL_DESCENDANTS"
  - "LEAF_CHILDREN"
  - "LEAF_DESCENDANTS"