tie break on multiple parents

proposals/1772-groups-as-rooms.md

@@ -148,8 +148,11 @@ relationship can be expressed in one of two ways:
 
     `canonical` determines whether this is the main parent for the space. When
     a user joins a room with a canonical parent, clients may switch to view
-    the room in the context of that parent space, peeking into it in order to
-    find other rooms and group them together.
+    the room in the context of that space, peeking into it in order to find
+    other rooms and group them together.  In practice, well behaved rooms
+    should only have one `canonical` parent, but given this is not enforced:
+    if multiple are present the client should select the one with the lowest
+    room ID, as determined via a lexicographic utf-8 ordering.
 
     To avoid abuse where a room admin falsely claims that a room is part of a
     space that it should not be, clients could ignore such `m.space.parent`