fix subscribe steps

Author: OrKoN

index.bs

@@ -763,7 +763,7 @@ Note: |navigables| is a set because a [=shared worker=] can be associated
 
 <div algorithm>
 
-The <dfn export>set of navigables for which an event is enabled</dfn>,
+The <dfn export>set of top-level traversables for which an event is enabled</dfn>,
 given |event name| and |session| is:
 
 1. Let |result| be a new [=/set=].
@@ -775,17 +775,21 @@ given |event name| and |session| is:
 
    1. If |subscription|'s is [=subscription/global=]:
 
-      1. For each |navigable| in remote end's [=/navigables=]:
+      1. For each |traversable| in remote end's [=/top-level traversables=]:
 
-        1. [=set/Append=] |navigable| to |result|.
+        1. [=set/Append=] |traversable| to |result|.
 
       1. Break.
 
    1. For each |context id| in |subscription|'s [=subscription/contexts=]:
 
       1. Let |navigable| be the result of [=trying=] to [=get a navigable=] with |context id|.
 
-      1. [=set/Append=] |navigable| to |result| if |navigable| is not null.
+      1. If |navigable| is null, continue.
+
+      1. Let |top-level traversable| be |navigable|'s [=navigable/top-level traversable=].
+
+      1. [=set/Append=] |top-level traversable| to |result| if |navigable| is not null.
 
 1. Return |result|.
 
@@ -1871,12 +1875,23 @@ The [=remote end steps=] with |session| and |command parameters| are:
    1. If the [=event=] with [=event name=] |event name| does not define [=remote end
      subscribe steps=], continue;
 
-   1. Let |existing navigables| be a [=set of navigables for which an event is enabled=] given |session| and |event name|.
+   1. Let |existing navigables| be a [=set of top-level traversables for which an event is enabled=] given |session| and |event name|.
+
+   1. Let |newly subscribed navigables| be a [=/set=].
+
+   1. If |context ids| is not empty:
+
+      1. For each |context id| in |context ids|:
+
+         1. Let |navigable| be the result of [=trying=] to [=get a navigable=] with |context id|.
+
+         1. If |navigable| is null, continue.
+
+         1. Let |top-level traversable| be |navigable|'s [=navigable/top-level traversable=].
 
-   1. Let |newly subscribed navigables| be all [=/navigables=] in the remote end if |context ids| is empty,
-      or navigables identified by ids in |context ids|.
+         1. [=set/Append=] |top-level traversable| to |newly subscribed navigables| if |top-level traversable| is not null.
 
-   TODO: clarify the above step.
+   1. Otherwise, |newly subscribed navigables| to a [=set=] of all [=navigable/top-level traversables=] in the remote end.
 
    1. Set |subscribe step events|[|event name|] to [=set/difference=] of
       |newly subscribed navigables| and |existing navigables|.