Accept m.space.parent links if there is a reverse link

proposals/1772-groups-as-rooms.md

@@ -151,10 +151,11 @@ relationship can be expressed in one of two ways:
 
     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`
-    events unless their sender has a sufficient power-level to send an
-    `m.space.child` event in the parent. The rationale for checking the power
-    level, rather than the *actual* presence of an `m.space.child` event in the
-    parent, is to accommodate "secret" rooms (see below).
+    events unless either (a) there is a corresponding `m.space.child` event in
+    the claimed parent, or (b) the sender of the `m.space.child` event has a
+    sufficient power-level to send such an `m.space.child` event in the parent.
+    [Checking the power-level rather than requiring an *actual* `m.space.child`
+    event in the parent allows for "secret" rooms (see below).]
 
     Where the parent space also claims a parent, clients can recursively peek
     into the grandparent space, and so on.