message: Column.ValueKind docs
diff: @@ -92,7 +92,13 @@ namespace Column {
  return !!v && !!(v as Column<any>).schema && !!(v as Column<any>).value;
  }
  - export const enum ValueKind { Present = 0, NotPresent = 1, Unknown = 2 }
  + export const enum ValueKind {
  + Present = 0,
  + /** Expressed in CIF as `.` */
  + NotPresent = 1,
  + /** Expressed in CIF as `?` */
  + Unknown = 2
  + }
  export function Undefined<T extends Schema>(rowCount: number, schema: T): Column<T['T']> {
  return constColumn(schema['T'], rowCount, schema, ValueKind.NotPresent);
  