sig string {}

sig User {
  login: string,
  auctions: set Auction,
  bids : set Bid
}

sig Auction {
 reserve : Int,
 startsOn : Int,
 endsOn : Int,
 item: one Item
}{
 one ~auctions
 // auction reserve cannot be negative
 reserve >= 0
 // auction cannot end before it has begun
 startsOn < endsOn
}

sig Bid {
 amount : Int,
 madeOn : Int,
 on : one Auction
}{
 one ~bids
 // bid amount cannot be negative
 amount >= 0
 // bid placed between auction start and end time
 madeOn >= on.startsOn
 madeOn <= on.endsOn
 // a User cannot bid on their own auction
 this.~@bids != on.~@auctions
}

sig Description {}

sig Item {
 description : Description
}{
 all disj a1,a2 : this.~item | a1.endsOn < a2.startsOn or a2.endsOn < a1.endsOn
}

run {} for 3 but exactly 2 Auction, exactly 2 Bid, 2 User, 1 Item