// Fill in the blanks to produce a syntactically valid, safe, and
// complete protocol
// The requirements for the desired protocol are stated in the exam

protocol OfferPayGoodsRefundWithdraw {
  roles Buyer, Seller
  parameters 
  private 

  Seller -> Buyer: offer [out ID, out item, out price]

  Buyer -> Seller: pay [in ID, in price, out pDone,       ]

  Seller -> Buyer: withdraw [in ID,            out rescind]

  Seller -> Buyer: goods [in ID, in item,        out gDone]

  Seller -> Buyer: refund [                               ]

  Buyer -> Seller: allDone [in ID,                        ]

  Buyer -> Seller: allDone [                   out outcome]

  Buyer -> Seller: allDone [                   out outcome]
}