6. Revocation how made.
A proposal is revoked-
(1) by the communication of notice of revocation by the proposer to the other party;
(2) by the lapse of the time prescribed in such proposal for its acceptance, or, if no time is so prescribed, by the lapse of a reasonable time, without communication of the acceptance;
(3) by the failure of the acceptor to fulfil a condition precedent to acceptance; or
(4) by the death or insanity of the proposer, if the fact of the death or insanity comes to the knowledge of the acceptor before acceptance.