validator(value) {
        const acceptableValues = [ ALL, BRANCH_PRIORITY, LEAF_PRIORITY, ALL_WITH_INDETERMINATE ]
        return includes(acceptableValues, value)
      }
Which kind of nodes should be included in the `value` array in multi-select mode.
Acceptable values:
  - "ALL" - Any node that is checked will be included in the `value` array
  - "BRANCH_PRIORITY" (default) - If a branch node is checked, all its descendants will be excluded in the `value` array
  - "LEAF_PRIORITY" - If a branch node is checked, this node itself and its branch descendants will be excluded from the `value` array but its leaf descendants will be included
  - "ALL_WITH_INDETERMINATE" - Any node that is checked will be included in the `value` array, plus indeterminate nodes