Add[left, right].
App3[right, left, center].
Result[add, app, weirdRep].

Add[left: #Number[], right: #Number[]]: #Add[a: left<Add, b: right<Add]
App3[center: #String[], left: #String[], right: #String[]]:
  #Append[a: left<App3, b: #Append[a: center<App3, b: right<App3]]

Result[
  add: Add[left: 2, right: 2]
  app: App3[left: "abc", center: "def", right: "ghi"]
  //Does `4 | 7`, but technically `5 | 6` (or `6 | 5` or `7 | 4`) are OK too.
  weirdRep: Add[left: 2 | 3, right: 2 | 4]
]?