The acceptance of such resignation shall not be necessary to make it effective, unless otherwise specified in the resignation. (rivma.org)
There are other templates as well like resignation letter templates, notice period letters, and notice to quit templates that can help you in writing such resignation letters. (template.net)
If the successor corporation assumes or substitutes outstanding awards held by a non-employee director and the non-employee director's status as a director of the Company or a director of the successor or acquiring company terminates other than upon voluntary resignation by the non-employee director (unless such resignation is at the request of the acquiror), then any options and stock appreciation rights held by the non-employee director will fully vest and become immediately exercisable. (sec.gov)