diff --git a/extra/oset/LICENSE.txt b/extra/oset/LICENSE.txt
new file mode 100644
index 000000000..aef85dda3
--- /dev/null
+++ b/extra/oset/LICENSE.txt
@@ -0,0 +1,29 @@
+License
+=======
+
+Copyright (c) 2009, Raymond Hettinger, and others
+All rights reserved.
+
+Package structured based on the one developed to odict
+Copyright (c) 2010, BlueDynamics Alliance, Austria
+
+
+* Redistributions of source code must retain the above copyright notice, this 
+  list of conditions and the following disclaimer.
+* Redistributions in binary form must reproduce the above copyright notice, this 
+  list of conditions and the following disclaimer in the documentation and/or 
+  other materials provided with the distribution.
+* Neither the name of the BlueDynamics Alliance nor the names of its 
+  contributors may be used to endorse or promote products derived from this 
+  software without specific prior written permission.
+      
+THIS SOFTWARE IS PROVIDED BY BlueDynamics Alliance ``AS IS`` AND ANY
+EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL BlueDynamics Alliance BE LIABLE FOR ANY
+DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/extra/oset/__init__.py b/extra/oset/__init__.py
new file mode 100644
index 000000000..688b31e92
--- /dev/null
+++ b/extra/oset/__init__.py
@@ -0,0 +1,3 @@
+"""Main Ordered Set module """
+
+from pyoset import oset
diff --git a/extra/oset/_abc.py b/extra/oset/_abc.py
new file mode 100644
index 000000000..b155c7474
--- /dev/null
+++ b/extra/oset/_abc.py
@@ -0,0 +1,475 @@
+#!/usr/bin/env python
+# -*- mode:python; tab-width: 2; coding: utf-8 -*-
+
+"""Partially backported python ABC classes"""
+
+from __future__ import absolute_import
+
+import sys
+import types
+
+if sys.version_info > (2, 6):
+    raise ImportError("Use native ABC classes istead of this one.")
+
+
+# Instance of old-style class
+class _C:
+    pass
+
+_InstanceType = type(_C())
+
+
+def abstractmethod(funcobj):
+    """A decorator indicating abstract methods.
+
+    Requires that the metaclass is ABCMeta or derived from it.  A
+    class that has a metaclass derived from ABCMeta cannot be
+    instantiated unless all of its abstract methods are overridden.
+    The abstract methods can be called using any of the normal
+    'super' call mechanisms.
+
+    Usage:
+
+        class C:
+            __metaclass__ = ABCMeta
+            @abstractmethod
+            def my_abstract_method(self, ...):
+                ...
+    """
+    funcobj.__isabstractmethod__ = True
+    return funcobj
+
+
+class ABCMeta(type):
+
+    """Metaclass for defining Abstract Base Classes (ABCs).
+
+    Use this metaclass to create an ABC.  An ABC can be subclassed
+    directly, and then acts as a mix-in class.  You can also register
+    unrelated concrete classes (even built-in classes) and unrelated
+    ABCs as 'virtual subclasses' -- these and their descendants will
+    be considered subclasses of the registering ABC by the built-in
+    issubclass() function, but the registering ABC won't show up in
+    their MRO (Method Resolution Order) nor will method
+    implementations defined by the registering ABC be callable (not
+    even via super()).
+
+    """
+
+    # A global counter that is incremented each time a class is
+    # registered as a virtual subclass of anything.  It forces the
+    # negative cache to be cleared before its next use.
+    _abc_invalidation_counter = 0
+
+    def __new__(mcls, name, bases, namespace):
+        cls = super(ABCMeta, mcls).__new__(mcls, name, bases, namespace)
+        # Compute set of abstract method names
+        abstracts = set(name
+                     for name, value in namespace.items()
+                     if getattr(value, "__isabstractmethod__", False))
+        for base in bases:
+            for name in getattr(base, "__abstractmethods__", set()):
+                value = getattr(cls, name, None)
+                if getattr(value, "__isabstractmethod__", False):
+                    abstracts.add(name)
+        cls.__abstractmethods__ = frozenset(abstracts)
+        # Set up inheritance registry
+        cls._abc_registry = set()
+        cls._abc_cache = set()
+        cls._abc_negative_cache = set()
+        cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
+        return cls
+
+    def register(cls, subclass):
+        """Register a virtual subclass of an ABC."""
+        if not isinstance(subclass, (type, types.ClassType)):
+            raise TypeError("Can only register classes")
+        if issubclass(subclass, cls):
+            return  # Already a subclass
+        # Subtle: test for cycles *after* testing for "already a subclass";
+        # this means we allow X.register(X) and interpret it as a no-op.
+        if issubclass(cls, subclass):
+            # This would create a cycle, which is bad for the algorithm below
+            raise RuntimeError("Refusing to create an inheritance cycle")
+        cls._abc_registry.add(subclass)
+        ABCMeta._abc_invalidation_counter += 1  # Invalidate negative cache
+
+    def _dump_registry(cls, file=None):
+        """Debug helper to print the ABC registry."""
+        print >> file, "Class: %s.%s" % (cls.__module__, cls.__name__)
+        print >> file, "Inv.counter: %s" % ABCMeta._abc_invalidation_counter
+        for name in sorted(cls.__dict__.keys()):
+            if name.startswith("_abc_"):
+                value = getattr(cls, name)
+                print >> file, "%s: %r" % (name, value)
+
+    def __instancecheck__(cls, instance):
+        """Override for isinstance(instance, cls)."""
+        # Inline the cache checking when it's simple.
+        subclass = getattr(instance, '__class__', None)
+        if subclass in cls._abc_cache:
+            return True
+        subtype = type(instance)
+        # Old-style instances
+        if subtype is _InstanceType:
+            subtype = subclass
+        if subtype is subclass or subclass is None:
+            if (cls._abc_negative_cache_version ==
+                ABCMeta._abc_invalidation_counter and
+                subtype in cls._abc_negative_cache):
+                return False
+            # Fall back to the subclass check.
+            return cls.__subclasscheck__(subtype)
+        return (cls.__subclasscheck__(subclass) or
+                cls.__subclasscheck__(subtype))
+
+    def __subclasscheck__(cls, subclass):
+        """Override for issubclass(subclass, cls)."""
+        # Check cache
+        if subclass in cls._abc_cache:
+            return True
+        # Check negative cache; may have to invalidate
+        if cls._abc_negative_cache_version < ABCMeta._abc_invalidation_counter:
+            # Invalidate the negative cache
+            cls._abc_negative_cache = set()
+            cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
+        elif subclass in cls._abc_negative_cache:
+            return False
+        # Check the subclass hook
+        ok = cls.__subclasshook__(subclass)
+        if ok is not NotImplemented:
+            assert isinstance(ok, bool)
+            if ok:
+                cls._abc_cache.add(subclass)
+            else:
+                cls._abc_negative_cache.add(subclass)
+            return ok
+        # Check if it's a direct subclass
+        if cls in getattr(subclass, '__mro__', ()):
+            cls._abc_cache.add(subclass)
+            return True
+        # Check if it's a subclass of a registered class (recursive)
+        for rcls in cls._abc_registry:
+            if issubclass(subclass, rcls):
+                cls._abc_cache.add(subclass)
+                return True
+        # Check if it's a subclass of a subclass (recursive)
+        for scls in cls.__subclasses__():
+            if issubclass(subclass, scls):
+                cls._abc_cache.add(subclass)
+                return True
+        # No dice; update negative cache
+        cls._abc_negative_cache.add(subclass)
+        return False
+
+
+def _hasattr(C, attr):
+    try:
+        return any(attr in B.__dict__ for B in C.__mro__)
+    except AttributeError:
+        # Old-style class
+        return hasattr(C, attr)
+
+
+class Sized:
+    __metaclass__ = ABCMeta
+
+    @abstractmethod
+    def __len__(self):
+        return 0
+
+    @classmethod
+    def __subclasshook__(cls, C):
+        if cls is Sized:
+            if _hasattr(C, "__len__"):
+                return True
+        return NotImplemented
+
+
+class Container:
+    __metaclass__ = ABCMeta
+
+    @abstractmethod
+    def __contains__(self, x):
+        return False
+
+    @classmethod
+    def __subclasshook__(cls, C):
+        if cls is Container:
+            if _hasattr(C, "__contains__"):
+                return True
+        return NotImplemented
+
+
+class Iterable:
+    __metaclass__ = ABCMeta
+
+    @abstractmethod
+    def __iter__(self):
+        while False:
+            yield None
+
+    @classmethod
+    def __subclasshook__(cls, C):
+        if cls is Iterable:
+            if _hasattr(C, "__iter__"):
+                return True
+        return NotImplemented
+
+Iterable.register(str)
+
+
+class Set(Sized, Iterable, Container):
+    """A set is a finite, iterable container.
+
+    This class provides concrete generic implementations of all
+    methods except for __contains__, __iter__ and __len__.
+
+    To override the comparisons (presumably for speed, as the
+    semantics are fixed), all you have to do is redefine __le__ and
+    then the other operations will automatically follow suit.
+    """
+
+    def __le__(self, other):
+        if not isinstance(other, Set):
+            return NotImplemented
+        if len(self) > len(other):
+            return False
+        for elem in self:
+            if elem not in other:
+                return False
+        return True
+
+    def __lt__(self, other):
+        if not isinstance(other, Set):
+            return NotImplemented
+        return len(self) < len(other) and self.__le__(other)
+
+    def __gt__(self, other):
+        if not isinstance(other, Set):
+            return NotImplemented
+        return other < self
+
+    def __ge__(self, other):
+        if not isinstance(other, Set):
+            return NotImplemented
+        return other <= self
+
+    def __eq__(self, other):
+        if not isinstance(other, Set):
+            return NotImplemented
+        return len(self) == len(other) and self.__le__(other)
+
+    def __ne__(self, other):
+        return not (self == other)
+
+    @classmethod
+    def _from_iterable(cls, it):
+        '''Construct an instance of the class from any iterable input.
+
+        Must override this method if the class constructor signature
+        does not accept an iterable for an input.
+        '''
+        return cls(it)
+
+    def __and__(self, other):
+        if not isinstance(other, Iterable):
+            return NotImplemented
+        return self._from_iterable(value for value in other if value in self)
+
+    def isdisjoint(self, other):
+        for value in other:
+            if value in self:
+                return False
+        return True
+
+    def __or__(self, other):
+        if not isinstance(other, Iterable):
+            return NotImplemented
+        chain = (e for s in (self, other) for e in s)
+        return self._from_iterable(chain)
+
+    def __sub__(self, other):
+        if not isinstance(other, Set):
+            if not isinstance(other, Iterable):
+                return NotImplemented
+            other = self._from_iterable(other)
+        return self._from_iterable(value for value in self
+                                   if value not in other)
+
+    def __xor__(self, other):
+        if not isinstance(other, Set):
+            if not isinstance(other, Iterable):
+                return NotImplemented
+            other = self._from_iterable(other)
+        return (self - other) | (other - self)
+
+    # Sets are not hashable by default, but subclasses can change this
+    __hash__ = None
+
+    def _hash(self):
+        """Compute the hash value of a set.
+
+        Note that we don't define __hash__: not all sets are hashable.
+        But if you define a hashable set type, its __hash__ should
+        call this function.
+
+        This must be compatible __eq__.
+
+        All sets ought to compare equal if they contain the same
+        elements, regardless of how they are implemented, and
+        regardless of the order of the elements; so there's not much
+        freedom for __eq__ or __hash__.  We match the algorithm used
+        by the built-in frozenset type.
+        """
+        MAX = sys.maxint
+        MASK = 2 * MAX + 1
+        n = len(self)
+        h = 1927868237 * (n + 1)
+        h &= MASK
+        for x in self:
+            hx = hash(x)
+            h ^= (hx ^ (hx << 16) ^ 89869747) * 3644798167
+            h &= MASK
+        h = h * 69069 + 907133923
+        h &= MASK
+        if h > MAX:
+            h -= MASK + 1
+        if h == -1:
+            h = 590923713
+        return h
+
+Set.register(frozenset)
+
+
+class MutableSet(Set):
+
+    @abstractmethod
+    def add(self, value):
+        """Add an element."""
+        raise NotImplementedError
+
+    @abstractmethod
+    def discard(self, value):
+        """Remove an element.  Do not raise an exception if absent."""
+        raise NotImplementedError
+
+    def remove(self, value):
+        """Remove an element. If not a member, raise a KeyError."""
+        if value not in self:
+            raise KeyError(value)
+        self.discard(value)
+
+    def pop(self):
+        """Return the popped value.  Raise KeyError if empty."""
+        it = iter(self)
+        try:
+            value = it.next()
+        except StopIteration:
+            raise KeyError
+        self.discard(value)
+        return value
+
+    def clear(self):
+        """This is slow (creates N new iterators!) but effective."""
+        try:
+            while True:
+                self.pop()
+        except KeyError:
+            pass
+
+    def __ior__(self, it):
+        for value in it:
+            self.add(value)
+        return self
+
+    def __iand__(self, it):
+        for value in (self - it):
+            self.discard(value)
+        return self
+
+    def __ixor__(self, it):
+        if not isinstance(it, Set):
+            it = self._from_iterable(it)
+        for value in it:
+            if value in self:
+                self.discard(value)
+            else:
+                self.add(value)
+        return self
+
+    def __isub__(self, it):
+        for value in it:
+            self.discard(value)
+        return self
+
+MutableSet.register(set)
+
+
+class OrderedSet(MutableSet):
+
+    def __init__(self, iterable=None):
+        self.end = end = []
+        end += [None, end, end]         # sentinel node for doubly linked list
+        self.map = {}                   # key --> [key, prev, next]
+        if iterable is not None:
+            self |= iterable
+
+    def __len__(self):
+        return len(self.map)
+
+    def __contains__(self, key):
+        return key in self.map
+
+    def __getitem__(self, key):
+        return list(self)[key]
+
+    def add(self, key):
+        if key not in self.map:
+            end = self.end
+            curr = end[PREV]
+            curr[NEXT] = end[PREV] = self.map[key] = [key, curr, end]
+
+    def discard(self, key):
+        if key in self.map:
+            key, prev, next = self.map.pop(key)
+            prev[NEXT] = next
+            next[PREV] = prev
+
+    def __iter__(self):
+        end = self.end
+        curr = end[NEXT]
+        while curr is not end:
+            yield curr[KEY]
+            curr = curr[NEXT]
+
+    def __reversed__(self):
+        end = self.end
+        curr = end[PREV]
+        while curr is not end:
+            yield curr[KEY]
+            curr = curr[PREV]
+
+    def pop(self, last=True):
+        if not self:
+            raise KeyError('set is empty')
+        key = reversed(self).next() if last else iter(self).next()
+        self.discard(key)
+        return key
+
+    def __repr__(self):
+        if not self:
+            return '%s()' % (self.__class__.__name__,)
+        return '%s(%r)' % (self.__class__.__name__, list(self))
+
+    def __eq__(self, other):
+        if isinstance(other, OrderedSet):
+            return len(self) == len(other) and list(self) == list(other)
+        return set(self) == set(other)
+
+    def __del__(self):
+        self.clear()                    # remove circular references
+
+if __name__ == '__main__':
+    print(OrderedSet('abracadaba'))
+    print(OrderedSet('simsalabim'))
diff --git a/extra/oset/pyoset.py b/extra/oset/pyoset.py
new file mode 100644
index 000000000..e77707d59
--- /dev/null
+++ b/extra/oset/pyoset.py
@@ -0,0 +1,82 @@
+#!/usr/bin/env python
+# -*- mode:python; tab-width: 2; coding: utf-8 -*-
+
+"""Partially backported python ABC classes"""
+
+from __future__ import absolute_import
+
+try:
+    from collections import MutableSet
+except ImportError:
+    # Running in Python <= 2.5
+    from ._abc import MutableSet
+
+
+KEY, PREV, NEXT = range(3)
+
+
+class OrderedSet(MutableSet):
+
+    def __init__(self, iterable=None):
+        self.end = end = []
+        end += [None, end, end]         # sentinel node for doubly linked list
+        self.map = {}                   # key --> [key, prev, next]
+        if iterable is not None:
+            self |= iterable
+
+    def __len__(self):
+        return len(self.map)
+
+    def __contains__(self, key):
+        return key in self.map
+
+    def __getitem__(self, key):
+        return list(self)[key]
+
+    def add(self, key):
+        if key not in self.map:
+            end = self.end
+            curr = end[PREV]
+            curr[NEXT] = end[PREV] = self.map[key] = [key, curr, end]
+
+    def discard(self, key):
+        if key in self.map:
+            key, prev, next = self.map.pop(key)
+            prev[NEXT] = next
+            next[PREV] = prev
+
+    def __iter__(self):
+        end = self.end
+        curr = end[NEXT]
+        while curr is not end:
+            yield curr[KEY]
+            curr = curr[NEXT]
+
+    def __reversed__(self):
+        end = self.end
+        curr = end[PREV]
+        while curr is not end:
+            yield curr[KEY]
+            curr = curr[PREV]
+
+    def pop(self, last=True):
+        if not self:
+            raise KeyError('set is empty')
+        key = reversed(self).next() if last else iter(self).next()
+        self.discard(key)
+        return key
+
+    def __repr__(self):
+        if not self:
+            return '%s()' % (self.__class__.__name__,)
+        return '%s(%r)' % (self.__class__.__name__, list(self))
+
+    def __eq__(self, other):
+        if isinstance(other, OrderedSet):
+            return len(self) == len(other) and list(self) == list(other)
+        return set(self) == set(other)
+
+    def __del__(self):
+        self.clear()                    # remove circular references
+
+oset = OrderedSet
diff --git a/lib/core/option.py b/lib/core/option.py
index d96d3faea..78be9ba50 100644
--- a/lib/core/option.py
+++ b/lib/core/option.py
@@ -23,6 +23,7 @@ import urlparse
 from extra.clientform.clientform import ParseResponse
 from extra.clientform.clientform import ParseError
 from extra.keepalive import keepalive
+from extra.oset.pyoset import oset
 from extra.xmlobject import xmlobject
 from lib.controller.checks import checkConnection
 from lib.core.common import Backend
@@ -1399,7 +1400,7 @@ def __setKnowledgeBaseAttributes(flushAll=True):
     if flushAll:
         kb.keywords = set(getFileItems(paths.SQL_KEYWORDS))
         kb.tamperFunctions = []
-        kb.targetUrls = set()
+        kb.targetUrls = oset()
         kb.testedParams = set()
         kb.userAgents = None
         kb.wordlist = None