Performance depends on terms, sec 601. Fiction of fulfilment of condition when party releases, sec 602. And so when such party prevents fulfilment, sec 603. And so when performance is waived, sec

604. Employee prevented by employer from working may recover, sec 605. Party disabling himself cannot set up technical default of other party, sec .

606. Substantial performance is sufficient, but this must be proved, sec 607. (Impossible condition vacates contract, sec 547.).