Ashley says to Bob: "I promise to pay you $20 if you shovel my sidewalk." Bob begins to shovel, and he soon completed 90% of the job. Then Ashley tells him: "Thanks, but I revoke." Bob then finishes the job in about ten minutes. Can Bob recover against Ashley in contract? Why or why not? In any event, what other theory might bob use? Assume that an offer for a unilateral contract is accepted by full performance of the requested act.