code: function unboxStrict(box, ifFrom) { // eslint-disable-line no-shadow
      const result = unbox(box, ifFrom, neverBoxed);
      if (result === neverBoxed) {
        throw new Error('Could not unbox');
      }
      return result;
    }
docs: Like unbox but raises an exception if unboxing fails.
  @param {*} box the box to unbox.
  @param {?function(function():boolean):boolean} ifFrom
  if the box may be opened by this unboxer's owner,
  then ifFrom receives the publicKey of the box creator.
  It should return true to allow unboxing to proceed.
  @return {*} the value if unboxing is allowed or fallback otherwise.