a) 'Past consideration is never good consideration.' Discuss.

b) Alysha wishes to celebrate her parents' 25th wedding anniversary. She contracted with Tom to construct a small pool for a pool party. Tom told her it was about a month's work and would cost 10,000. Alysha accepted Tom's price and they further agreed that the work would be completed before 1st December. Tom began work on 1st November but bad weather caused many delays. As a result, Tom said he would only be able to complete the job by 1st December if he employed extra workmen and this would increase his costs. Alysha offered to pay him 12,000 if he completed the job on time. Tom went on to complete the construction before 1st December and Alysha paid him 10,000 but refused to pay any more. Advise Tom.

Please describe up to one thousand five hundred words. Please insert relevant cases with authorities. Thank you.