6. Revocation how made.- A proposal is revoked-
(1) by the communication of notice of revocation by the pro poser 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 his death or insanity comes to the knowledge of the acceptor before acceptance.