1  export type Optional<T> = T | None;
 2  export type None = null | undefined;
 3  
 4  /**
 5   * Determine if an optional value is present.
 6   *
 7   * @param optional value
 8   * @return true if present, false otherwise
 9   */
10  export function isSome<T>(optional: Optional<T>): optional is T {
11      return optional !== null && optional !== undefined;
12  }
13  
14  /**
15   * Determine if an optional value is not present.
16   *
17   * @param optional value
18   * @return true if not present, false otherwise
19   */
20  export function isNone<T>(optional: Optional<T>): optional is None {
21      return optional === null || optional === undefined;
22  }