Using what you have learned, how would you model the relationship between a buyer and their sales transactions? What would you add and to what schema? What would be the impact on the underlying tables?
Keeping in mind that every order has a single buyer, but every buyer (Recipient) can have multiple orders, you would define the link in the order schema. Most relationships are 1:1 in one direction and 1:M in the reverse direction. You define the link in the 1:1 "direction". This has been done with neo:order.
To define a true M:M relationship you have to define a table to hold each relationship. This has been done with the xtk:rights schema that is used to configure the Operator rights on folders. Each entry in this table has a link to an Operator or Operator Group and a link to a Folder:
<element desc="Right associated to folder" label="Folder" name="folder" revIntegrity="own"