Add OS difference reporting to Ndiff. It looks like

Remove OS "Linux 2.6.18 - 2.6.25".
        Remove OS "Linux 2.6.8 - 2.6.20".
        Remove OS "OpenWrt (Linux 2.6.19 - 2.6.21)".
        Add OS "Archos 605 WiFi video player".
        Add OS "Linux 2.6.27 (Ubuntu)".
        Add OS "Linux 2.6.22".
        Add OS "Linux 2.6.27".

ndiff/docs/ndiff.dtd

@@ -109,6 +109,18 @@ The host had a hostname in the A scan that it didn't have in the B scan.
 -->
 <!ELEMENT host-hostname-remove (hostname)>
 
+<!--
+The host gained an OS in the B scan that it didn't have in the A scan.
+-->
+<!ELEMENT host-os-add EMPTY>
+<!attlist host-os-add name #REQUIRED>
+
+<!--
+The host had an OS in the A scan that it didn't have in the B scan.
+-->
+<!ELEMENT host-os-remove EMPTY>
+<!attlist host-os-remove name #REQUIRED>
+
 <!--
 The services that were running on two ports were swapped between the A
 and B scans. The portid and protocol attributes give the A and B port

ndiff/ndiff

@@ -313,6 +313,34 @@ class HostHostnameRemoveHunk(DiffHunk):
         elem.appendChild(address_elem)
         return frag
 
+class HostOsAddHunk(DiffHunk):
+    def __init__(self, os):
+        self.os = os
+
+    def to_string(self):
+        return u"Add OS \"%s\"." % self.os
+
+    def to_dom_fragment(self, document):
+        frag = document.createDocumentFragment()
+        elem = document.createElement(u"host-os-add")
+        elem.setAttribute(u"name", self.os)
+        frag.appendChild(elem)
+        return frag
+
+class HostOsRemoveHunk(DiffHunk):
+    def __init__(self, os):
+        self.os = os
+
+    def to_string(self):
+        return u"Remove OS \"%s\"." % self.os
+
+    def to_dom_fragment(self, document):
+        frag = document.createDocumentFragment()
+        elem = document.createElement(u"host-os-remove")
+        elem.setAttribute(u"name", self.os)
+        frag.appendChild(elem)
+        return frag
+
 class PortIdChangeHunk(DiffHunk):
     def __init__(self, a_spec, b_spec):
         self.a_spec = a_spec
@@ -531,6 +559,12 @@ def host_diff(a, b):
     all_specs.sort()
     for spec in all_specs:
         diff.extend(port_diff(a.ports[spec], b.ports[spec]))
+
+    for os in set(a.os) - set(b.os):
+        diff.append(HostOsRemoveHunk(os))
+    for os in set(b.os) - set(a.os):
+        diff.append(HostOsAddHunk(os))
+
     return diff
 
 def scan_diff(a, b):