diff -r 893c55cd168d core.startup/nbproject/project.xml
--- a/core.startup/nbproject/project.xml	Wed Aug 05 06:31:25 2009 +0400
+++ b/core.startup/nbproject/project.xml	Wed Aug 05 14:26:46 2009 +0200
@@ -60,7 +60,7 @@
                     <build-prerequisite/>
                     <compile-dependency/>
                     <run-dependency>
-                        <specification-version>7.19</specification-version>
+                        <specification-version>7.23</specification-version>
                     </run-dependency>
                 </dependency>
                 <dependency>
diff -r 893c55cd168d core.startup/src/org/netbeans/core/startup/layers/ModuleLayeredFileSystem.java
--- a/core.startup/src/org/netbeans/core/startup/layers/ModuleLayeredFileSystem.java	Wed Aug 05 06:31:25 2009 +0400
+++ b/core.startup/src/org/netbeans/core/startup/layers/ModuleLayeredFileSystem.java	Wed Aug 05 14:26:46 2009 +0200
@@ -137,8 +137,12 @@
         List<FileSystem> l = new ArrayList<FileSystem>(fs2s.length + 2);
         l.add(fs1);
         if (addLookup) {
-            Collection<? extends FileSystem> fromLookup = result.allInstances();
-            l.addAll(fromLookup);
+            for (FileSystem f : result.allInstances()) {
+                if (Boolean.TRUE.equals(f.getRoot().getAttribute("fallback"))) { // NOI18N
+                    continue;
+                }
+                l.add(f);
+            }
         }
         l.addAll(Arrays.asList(fs2s));
         l.add(fs3);
@@ -175,6 +179,13 @@
                 err.log(Level.WARNING, "Setting layer URLs: " + layerUrls, x);
             }
         }
+        if (addLookup) {
+            for (FileSystem f : result.allInstances()) {
+                if (Boolean.TRUE.equals(f.getRoot().getAttribute("fallback"))) { // NOI18N
+                    l.add(f);
+                }
+            }
+        }
         return l.toArray(new FileSystem[l.size()]);
     }
 
diff -r 893c55cd168d core.startup/test/unit/src/org/netbeans/core/startup/layers/SystemFileSystemTest.java
--- a/core.startup/test/unit/src/org/netbeans/core/startup/layers/SystemFileSystemTest.java	Wed Aug 05 06:31:25 2009 +0400
+++ b/core.startup/test/unit/src/org/netbeans/core/startup/layers/SystemFileSystemTest.java	Wed Aug 05 14:26:46 2009 +0200
@@ -63,6 +63,7 @@
 import org.openide.filesystems.FileRenameEvent;
 import org.openide.filesystems.FileSystem;
 import org.openide.filesystems.FileUtil;
+import org.openide.filesystems.LocalFileSystem;
 import org.openide.modules.ModuleInfo;
 import org.openide.util.Lookup;
 import org.openide.util.lookup.InstanceContent;
@@ -230,6 +231,63 @@
         assertTrue("Still valid", global.isValid());
         assertEquals("fileone", read(global));
     }
+
+    public void testDynamicSystemsCanAlsoBeBehindLayers() throws Exception {
+        ModuleManager mgr = Main.getModuleSystem ().getManager();
+        mgr.mutexPrivileged().enterWriteAccess();
+        Module m1 = mgr.create(new File(jars, "base-layer-mod.jar"), null, false, false, false);
+        FileObject global;
+        try {
+            assertEquals(Collections.EMPTY_SET, m1.getProblems());
+            mgr.enable(m1);
+            global = FileUtil.getConfigFile("foo/file2.txt");
+            assertNotNull("File Object installed: " + global, global);
+            assertEquals("base contents", read(global));
+
+            final LocalFileSystem lfs1 = new LocalFileSystem();
+            File dir1 = new File(getWorkDir(), "dir1");
+            dir1.mkdirs();
+            lfs1.setRootDirectory(dir1);
+            lfs1.getRoot().setAttribute("fallback", Boolean.TRUE);
+            assertEquals("Position attribute is there", -1, lfs1.getRoot().getAttribute("fallback"));
+            fs1 = lfs1;
+
+            FileObject fo1 = FileUtil.createData(fs1.getRoot(), global.getPath());
+            fo1.setAttribute("one", 1);
+            write(fo1, "fileone");
+
+            FileObject fo11 = FileUtil.createData(fs1.getRoot(), "test-fs-is-there.txt");
+            write(fo11, "hereIam");
+
+            FileObject fo2 = FileUtil.createData(fs2.getRoot(), global.getPath());
+            fo2.setAttribute("two", 2);
+            write(fo2, "two");
+
+            events.clear();
+
+
+            MainLookup.register(fs1, this);
+            MainLookup.register(fs2, this);
+
+            Iterator<? extends FileSystem> it = Lookup.getDefault().lookupAll(FileSystem.class).iterator();
+            assertTrue("At least One", it.hasNext());
+            assertEquals("first is fs1", fs1, it.next());
+            assertTrue("At least two ", it.hasNext());
+            assertEquals("first is fs2", fs2, it.next());
+
+            assertEquals("two", read(global));
+        } finally {
+            mgr.disable(m1);
+            mgr.delete(m1);
+            mgr.mutexPrivileged().exitWriteAccess();
+        }
+        assertTrue("Still valid", global.isValid());
+        assertEquals("two", read(global));
+
+        FileObject fo = FileUtil.getConfigFile("test-fs-is-there.txt");
+        assertNotNull("File found", fo);
+        assertEquals("Text is correct", "hereIam", read(fo));
+    }
     
     public void testCanHideFilesFromModules() throws Exception {
         ModuleManager mgr = Main.getModuleSystem ().getManager();
diff -r 893c55cd168d openide.filesystems/apichanges.xml
--- a/openide.filesystems/apichanges.xml	Wed Aug 05 06:31:25 2009 +0400
+++ b/openide.filesystems/apichanges.xml	Wed Aug 05 14:26:46 2009 +0200
@@ -46,7 +46,25 @@
         <apidef name="filesystems">Filesystems API</apidef>
     </apidefs>
     <changes>
-        <change id="FileObject.asText">
+         <change id="add-fallback-content-to-sfs">
+            <api name="filesystems"/>
+            <summary>Allow modules to dynamically add/remove layer content</summary>
+            <version major="7" minor="23"/>
+            <date day="12" month="8" year="2009"/>
+            <author login="jtulach"/>
+            <compatibility addition="yes" binary="compatible" source="compatible" semantic="compatible" deprecation="no" deletion="no" modification="no"/>
+            <description>
+                <p>
+                    One can provide <q>fallback</q>
+                    <a href="@TOP@/architecture-summary.html#answer-arch-usecases">content of system filesystem</a> by
+                    returning <code>Boolean.TRUE</code> from call
+                    to <code>fs.getRoot().getAttribute("fallback")</code>.
+                </p>
+            </description>
+            <class package="org.openide.filesystems" name="Repository"/>
+            <issue number="169892"/>
+        </change>
+       <change id="FileObject.asText">
             <api name="filesystems"/>
             <summary>Read files with asText(), asBytes() and asLines()</summary>
             <version major="7" minor="21"/>
diff -r 893c55cd168d openide.filesystems/arch.xml
--- a/openide.filesystems/arch.xml	Wed Aug 05 06:31:25 2009 +0400
+++ b/openide.filesystems/arch.xml	Wed Aug 05 14:26:46 2009 +0200
@@ -370,7 +370,7 @@
     is provided for applications where a history of recently used directories is a
     security concern.
     </li>
-    </ul>
+</ul>
 </answer>
 
 
@@ -909,6 +909,16 @@
           <a href="@TOP@/org/openide/filesystems/FileUtil.html#createMemoryFileSystem()">memory file system</a>,
           or any other you write yourself.
       </p>
+
+      <p>
+          Since version 7.23 you can use <api name="SystemFileSystem.fallback" group="property"
+          type="export" category="stable">
+              By returning <code>Boolean.TRUE</code> from call
+                    to <code>fs.getRoot().getAttribute("fallback")</code>
+                    you can place your filesystem behind all layers provided
+                    by standard modules and form a kind of <q>fallback</q>
+         </api>
+      </p>
   </usecase>
  </answer>
 
diff -r 893c55cd168d openide.filesystems/nbproject/project.properties
--- a/openide.filesystems/nbproject/project.properties	Wed Aug 05 06:31:25 2009 +0400
+++ b/openide.filesystems/nbproject/project.properties	Wed Aug 05 14:26:46 2009 +0200
@@ -44,4 +44,4 @@
 javadoc.main.page=org/openide/filesystems/doc-files/api.html
 javadoc.arch=${basedir}/arch.xml
 javadoc.apichanges=${basedir}/apichanges.xml
-spec.version.base=7.22.0
+spec.version.base=7.23.0