Does spicedb support

exist

/`not_exist` operator to check if the relationship exist? This is useful to implement conditional rules that's based on the existence of the relationship data. for instance:

definition document {
  relation approver: user
  relation reviewer: user
  relation commenter: user

  permission read = exist(approver) & reviewer + not_exist(approver) & commenter
}

This is an example to express different read permissions based on whether a document has been approved or not