6. Revocation, how made

A proposal is revoked by —

the communication of notice of revocation by the proposer to the other party;

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;

the failure of the acceptor to fulfil a condition precedent to acceptance;

the death or mental disorder of the proposer, if the fact of his death or mental disorder comes to the knowledge of the acceptor before acceptance.