Fix and clarify exposed dfn (#464)

index.bs

@@ -8645,12 +8645,18 @@ is the [=set/intersection=] of the relevant construct's [=own exposure set=]
 with the the [=host interface=]'s [=exposure set=].
 Otherwise, it is the [=host interface=]'s [=exposure set=].
 
-An [=interface=], [=namespace=], [=interface member=], or [=namespace member=] is
-<dfn id="dfn-exposed" export>exposed</dfn> in a given ECMAScript global environment if the
-ECMAScript global object implements an interface that is in the construct's [=exposure set=], and either:
-
-*   the [=relevant settings object=] for the ECMAScript global object is a [=secure context=]; or
-*   the construct is not [=available only in secure contexts=].
+<div algorithm>
+    An [=interface=], [=namespace=], or [=member=] |construct| is <dfn id="dfn-exposed" export>exposed</dfn>
+    in a given [=Realm=] |realm| if the following steps return true:
+
+    1.  If |realm|.\[[GlobalObject]] does not implement an [=interface=]
+        that is in |construct|'s [=exposure set=], then return false.
+    1.  If |construct| is [=available in both secure and non-secure contexts=],
+        then return true.
+    1.  If the [=relevant settings object=] of |realm|.\[[GlobalObject]] is a [=secure context=],
+        then return true.
+    1.  Otherwise, return false.
+</div>
 
 Note: Since it is not possible for the [=relevant settings object=]
 for an ECMAScript global object to change whether it is a