It's a thing which can be added to a real-world contract. "Buyer agrees as an unalterable condition that: if Buyer or Buyer's estate sells, gifts, donates, or

It's a thing which can be added to a real-world contract. "Buyer agrees as an unalterable condition that: if Buyer or Buyer's estate sells, gifts, donates, or transfers the Artifact in any way, Two Percent of the Current Market Value of the Artifact is due to the Artist or the Artist's Estate within 30 days of such sale, gift, donation, or transfer. The Current Market Value is the higher of the most recent price paid for the Artifact or an appraisal made by an independent, reasonable expert up to 1 Year prior to the sale, gift, donation or transfer." Or the like, as adjusted for jurisdictions.