old_contents: iD.Relation = iD.Entity.extend({
      type: "relation",
      members: [],
  
      extent: function() {
          return [[NaN, NaN], [NaN, NaN]];
      }
  });
  
new_contents: iD.Relation = iD.Entity.extend({
      type: "relation",
      members: [],
  
      extent: function() {
          return [[NaN, NaN], [NaN, NaN]];
      },
  
      geometry: function() {
          return 'relation';
      }
  });
  
subject: Define Relation#geometry