diff --git a/.gitattributes b/.gitattributes
index 5446f0b9009..1f4188a473b 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -3485,9 +3485,11 @@ Ridges_3/examples/Ridges_3/skip_vcproj_auto_generation -text
 Ridges_3/test/Ridges_3/data/ellipsoid.off -text svneol=unset#application/octet-stream
 Robustness/demo/Robustness/help/index.html svneol=native#text/html
 Robustness/demo/Robustness/robustness.vcproj eol=crlf
+STL_Extension/doc_tex/STL_Extension_ref/Uncertain.tex -text
 STL_Extension/include/CGAL/Default_argument.h -text
 STL_Extension/include/CGAL/array.h -text
 STL_Extension/include/CGAL/type_traits.h -text
+STL_Extension/test/STL_Extension/test_Uncertain.cpp -text
 STL_Extension/test/STL_Extension/test_type_traits.cpp -text
 Scripts/developer_scripts/autotest_cgal_with_cmake -text
 Scripts/developer_scripts/check_no_CGAL_USE_without_includes_before -text
diff --git a/STL_Extension/doc_tex/STL_Extension/main.tex b/STL_Extension/doc_tex/STL_Extension/main.tex
index 6b83b4340c7..ce61555fa31 100644
--- a/STL_Extension/doc_tex/STL_Extension/main.tex
+++ b/STL_Extension/doc_tex/STL_Extension/main.tex
@@ -26,7 +26,7 @@ generic algorithms, iterators, functor adaptors for binding and swapping
 arguments and for composition, functors for projection and creation and
 adaptor classes around iterators and circulators. See also circulators in
 Chapter~\ref{chapterCirculators}.  A class storing polymorphic objects
-is also provided.
+is also provided, as well as a class to manage the uncertainty of some values.
 
 \section{Doubly-Connected List Managing Items in Place}
 
@@ -112,3 +112,23 @@ It can be used by a function to return objects of different types.
 A mechanism to extract the stored object based on its type is also provided.
 This class is similar to \ccc{boost::any}.
 
+\section{Uncertainty management}
+
+The class \ccStyle{Uncertain<T>} represents a range of values of type \ccc{T}.
+\ccc{T} is allowed to stand for \ccc{bool}, or CGAL's enumeration types
+\ccc{Sign}, \ccc{Comparison_result}, \ccc{Orientation}, \ccc{Oriented_side},
+\ccc{Bounded_side} and \ccc{Angle}.
+
+The idea is that sometimes you are not sure of the result of a function,
+and you would like to communicate that to the caller.  \ccc{Uncertain<T>}
+allows just that.  It also provides functions to naturally extend the
+Boolean operations for \ccc{Uncertain<bool>} for example.
+
+\ccc{Uncertain<T>} is used in CGAL as the return type of geometric predicates
+when the number type used is interval arithmetic like \ccc{Interval_nt}.
+End users typically do not see it as it is hidden in the implementation
+of the filtered predicates provided by the various filtered kernels,
+but it is important that providers of predicates that are meant to be
+filtered by \ccc{Filtered_predicate}, know about it.
+
+It can also be used in other contexts as well, as it is a general tool.
diff --git a/STL_Extension/doc_tex/STL_Extension_ref/Uncertain.tex b/STL_Extension/doc_tex/STL_Extension_ref/Uncertain.tex
new file mode 100644
index 00000000000..f1f12283a0d
--- /dev/null
+++ b/STL_Extension/doc_tex/STL_Extension_ref/Uncertain.tex
@@ -0,0 +1,326 @@
+%% =============================================================================
+%% The CGAL Reference Manual
+%% Chapter: STL Extensions - The Reference Part
+%% -----------------------------------------------------------------------------
+%% author: Sylvain Pion
+%% -----------------------------------------------------------------------------
+%% $Id: $
+%% $URL: $
+%% =============================================================================
+
+%% +=========================================================================+
+
+\begin{ccRefClass}{Uncertain<T>}
+
+\ccDefinition
+
+An object of the class \ccClassTemplateName\ represents an uncertainty on the
+value of type \ccc{T}.  This uncertainty is represented by a non-empty range of
+values of type \ccc{T}.
+
+The idea is that sometimes you are not sure of the result of a function, and
+you would like to communicate that to the caller.  \ccc{Uncertain<T>} allows
+just that.
+
+\ccc{Uncertain<T>} is also meant to be used as a drop-in replacement for
+\ccc{T} in some template contexts, as much as possible.  This is why it
+provides overloaded operators and functions to naturally extend the Boolean
+operations for \ccc{Uncertain<bool>} for example, or the operations on
+enumeration types.
+
+\ccc{Uncertain<T>} is used in CGAL as the return type of geometric predicates
+when the number type used is interval arithmetic like \ccc{Interval_nt}.  End
+users typically do not see it, as it is hidden in the implementation of the
+filtered predicates provided by the various filtered kernels, but it is
+important that providers of predicates that are meant to be filtered by
+\ccc{Filtered_predicate}, know about it.
+
+Note concerning \cgal\ assertions: assertions checking an expression of type
+\ccc{Uncertain<bool>} will trigger an assertion failure only if the assertion
+is certainly false.  In case of an indeterminate value, the assertion is not
+triggered.  This means that we assume, in case of doubt, that there is no
+error.
+
+It can also be used in other contexts as well, as it is a general tool.
+This can be seem as support for non-deterministic programming.
+Finally, note that this class has some common points with \ccc{boost::tribool}.
+
+  \ccInclude{CGAL/Uncertain.h}
+
+  %% +-----------------------------------+
+  \ccParameters
+
+  The parameter \ccStyle{T} can either be \ccc{bool} or one of the three-valued
+  (-1, 0, 1) enumeration types: \ccc{Sign}, \ccc{Comparison_result},
+  \ccc{Orientation}, \ccc{Oriented_side}, \ccc{Bounded_side} or \ccc{Angle}.
+
+  Some functions are defined only when \ccc{T} is \ccc{bool} or alternatively
+  when it is one of the enumeration types listed previously.
+
+% TODO : the CGALi::Minmax_traits should be documented (and renamed before that).
+
+  %% +-----------------------------------+
+  \ccTypes
+  \ccSetThreeColumns{static Uncertain<T>}{l.swap( l1)xxxxxxxxxxxx;}{}
+  \ccPropagateThreeToTwoColumns
+
+  \ccNestedType{value_type}{The type \ccc{T}.}
+
+  \ccNestedType{Uncertain_conversion_exception}{The type of the exception
+  thrown for uncertain conversions.  It is a typedef to the type
+  \ccc{CGAL::Uncertain_conversion_exception} which derives from \ccc{std::range_error}.}
+
+  %% +-----------------------------------+
+  \ccCreation
+  \ccCreationVariable{u}
+
+  \ccConstructor{Uncertain();}
+  {introduces an indeterminate value.}
+
+  \ccConstructor{Uncertain(T t);}
+  {introduces a certain object with value \ccc{t}.}
+
+  \ccMethod{Uncertain& operator=(T t);}
+  {assigns the certain value \ccc{t} to \ccc{u}.}
+
+  \ccConstructor{Uncertain(T i, T s);}
+  {introduces an object representing the range with lower bound \ccc{i} and
+  upper bound \ccc{s}. \ccPrecond $ i<= s$. }
+
+  %% +-----------------------------------+
+  \ccHeading{Access Functions}
+
+  % \def\ccTagRmTrailingConst{\ccFalse}
+  \ccMethod{T inf() const;}
+  {returns the lower bound of the range represented by~\ccVar.}
+
+  \ccGlue\ccMethod{T sup() const;}
+  {returns the upper bound of the range represented by~\ccVar.}
+
+  %% +-----------------------------------+
+  \ccHeading{Uncertainty testing and conversion}
+
+  \ccMethod{bool is_certain() const;}
+  {returns \ccc{true} iff the value is certain, that is, it is unique, the range
+   is a singleton. That is, \ccc{u.inf() == u.sup()}.}
+
+  \ccMethod{T make_certain() const;}
+  {if \ccVar.\ccc{is_certain()}, then returns the certain value which is represented.
+   Otherwise, throws an exception of type \ccc{Uncertain_conversion_exception}.}
+
+  \ccMethod{operator T() const;}
+  {conversion operator to \ccc{T}.  It does and returns the same thing as
+   \ccVar.\ccc{make_certain()}.  Note that relying on the automatic conversion
+   can throw exceptions, which defeats the purpose of propagating uncertainty.
+   Nevertheless, in many cases, it is hard to avoid it, for example for the
+   \ccc{&&} and $||$ operators for \ccc{bool} (see below).}
+
+  %% +-----------------------------------+
+  \ccHeading{Static member function}
+
+  \def\ccTagRmEigenClassName{\ccFalse}
+  \ccFunction{static Uncertain<T> Uncertain<T>::indeterminate();}
+  {returns an indeterminate range, like the default constructor.}
+
+  %% +-----------------------------------+
+  \ccHeading{Free functions}
+
+  \ccFunction{template <class T> T inf(Uncertain<T> u);}
+  {returns \ccc{u.inf()}.}
+
+  \ccFunction{template <class T> T sup(Uncertain<T> u);}
+  {returns \ccc{u.sup()}.}
+
+  \ccFunction{template <class T> bool is_certain(T t);}
+  {returns \ccc{true}.}
+
+  \ccFunction{template <class T> bool is_certain(Uncertain<T> u);}
+  {returns \ccc{u}.\ccc{is_certain}().}
+
+  \ccFunction{template <class T> bool is_indeterminate(T u);}
+  {returns \ccc{false}.}
+
+  \ccFunction{template <class T> bool is_indeterminate(Uncertain<T> u);}
+  {returns \ccc{!is_certain(u)}.}
+
+  \ccFunction{template <class T> T get_certain(T t);}
+  {returns \ccc{t}.}
+
+  \ccFunction{template <class T> T get_certain(Uncertain<T> u);}
+  {returns \ccc{u}.\ccc{make_certain}().  \ccPrecond \ccc{u}.\ccc{is_certain}().}
+
+  \ccFunction{template <class T> T make_certain(T t);}
+  {returns \ccc{t}.}
+
+  \ccFunction{template <class T> T make_certain(Uncertain<T> u);}
+  {returns \ccc{u}.\ccc{make_certain}().}
+
+  \ccFunction{template <class T> Uncertain<T> make_uncertain(T t);}
+  {returns \ccc{Uncertain<T>(u)}.}
+
+  \ccFunction{template <class T> Uncertain<T> make_uncertain(Uncertain<T> u);}
+  {returns \ccc{u}.}
+
+  %% +-----------------------------------+
+  \ccHeading{Overloaded operators}
+
+  The overloaded operators and functions are defined as preserving the set-inclusion property.
+  Similarly to interval arithmetic, the returned range is guaranteed to contain
+  the result of the operation over all values of the input range(s).
+  In the following documentation we express this as the extension of the corresponding function
+  over the type \ccc{T}.
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator==(Uncertain<T> u, Uncertain<T> v);}
+  {returns the extension of the equality operator over \ccc{u} and \ccc{v}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator==(Uncertain<T> u, T v);}
+  {returns \ccc{u == make_uncertain(v)}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator==(T u, Uncertain<T> v);}
+  {returns \ccc{v == u}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator!=(Uncertain<T> u, Uncertain<T> v);}
+  {returns the extension of the inequality operator over \ccc{u} and \ccc{v}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator!=(Uncertain<T> u, T v);}
+  {returns \ccc{u != make_uncertain(v)}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator!=(T u, Uncertain<T> v);}
+  {returns \ccc{v != u}.}
+
+  %% +-----------------------------------+
+  \ccHeading{Overloaded operators for \ccc{Uncertain<bool>} only}
+
+  \ccFunction{Uncertain<bool> operator!(Uncertain<bool> u);}
+  {returns the range containing the negated values of \ccc{u}.}
+
+  \ccFunction{Uncertain<bool> operator|(Uncertain<bool> u, Uncertain<bool> v);}
+  {returns the range containing the values computed as logical or from \ccc{u} and \ccc{v}.}
+
+  \ccFunction{Uncertain<bool> operator|(Uncertain<bool> u, bool v);}
+  {returns \ccc{u | make_uncertain(v)}.}
+
+  \ccFunction{Uncertain<bool> operator|(bool u, Uncertain<bool> v);}
+  {returns \ccc{v | u}.}
+
+  \ccFunction{Uncertain<bool> operator&(Uncertain<bool> u, Uncertain<bool> v);}
+  {returns the range containing the values computed as logical and from \ccc{u} and \ccc{v}.}
+
+  \ccFunction{Uncertain<bool> operator&(Uncertain<bool> u, bool v);}
+  {returns \ccc{u & make_uncertain(v)}.}
+
+  \ccFunction{Uncertain<bool> operator&(bool u, Uncertain<bool> v);}
+  {returns \ccc{v & u}.}
+
+  Note : the logical operators $\&\&$ and $||$ are not overloaded on purpose.  The reason
+  is that, when \ccc{f() && g()} is evaluated and they return \ccc{bool}, then \ccc{g()}
+  is only evaluated when \ccc{f()} returns \ccc{true}.  One could have a dependency so
+  that \ccc{g()} has an internal precondition that implies that \ccc{f()} returns true.
+  The overloaded operators for user-defined types do not provide this short-circuiting
+  property, and so, if the overloaded operators where provided, then \ccc{g()} would
+  be evaluated, no matter the result of \ccc{f()}, which could lead to an unwanted
+  situation.  The $\&$ and $|$ operators do not have this short-circuiting property,
+  and are therefore overloaded safely.
+
+  %% +-----------------------------------+
+  \ccHeading{Overloaded operators and functions for \ccc{Uncertain<enum T>} only}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator<(Uncertain<T> u, Uncertain<T> v);}
+  {returns the extension of the less-than operator over \ccc{u} and \ccc{v}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator<(Uncertain<T> u, T v);}
+  {returns \ccc{u < make_uncertain(v)}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator<(T u, Uncertain<T> v);}
+  {returns \ccc{make_uncertain(u) < v}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator>(Uncertain<T> u, Uncertain<T> v);}
+  {returns the extension of the greater-than operator over \ccc{u} and \ccc{v}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator>(Uncertain<T> u, T v);}
+  {returns \ccc{u > make_uncertain(v)}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator>(T u, Uncertain<T> v);}
+  {returns \ccc{make_uncertain(u) > v}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator<=(Uncertain<T> u, Uncertain<T> v);}
+  {returns the extension of the less-than or equal operator over \ccc{u} and \ccc{v}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator<=(Uncertain<T> u, T v);}
+  {returns \ccc{u <= make_uncertain(v)}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator<=(T u, Uncertain<T> v);}
+  {returns \ccc{make_uncertain(u) <= v}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator>=(Uncertain<T> u, Uncertain<T> v);}
+  {returns the extension of the greater-than or equal operator over \ccc{u} and \ccc{v}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator>=(Uncertain<T> u, T v);}
+  {returns \ccc{u > make_uncertain(v)}.}
+
+  \ccFunction{template <class T>
+              Uncertain<bool> operator>=(T u, Uncertain<T> v);}
+  {returns \ccc{make_uncertain(u) >= v}.}
+
+  \ccFunction{template <class T>
+              Uncertain<T> operator*(Uncertain<T> u, Uncertain<T> v);}
+  {returns the extension of the multiplication operator over \ccc{u} and \ccc{v}.
+   This requires \ccc{T} to have a multiplication operator as well.}
+
+  \ccFunction{template <class T>
+              Uncertain<T> operator*(Uncertain<T> u, T v);}
+  {returns \ccc{u * make_uncertain(v)}.}
+
+  \ccFunction{template <class T>
+              Uncertain<T> operator<(T u, Uncertain<T> v);}
+  {returns \ccc{make_uncertain(u) * v}.}
+
+  \ccFunction{template <class T>
+              Uncertain<T> operator-(Uncertain<T> u);}
+  {returns the extension of the unary minus operator over \ccc{u}.}
+
+  \ccFunction{template <class T, class U>
+              Uncertain<T> enum_cast(Uncertain<U> u);}
+  {returns the extension of the \ccc{enum_cast<T>} function over \ccc{u}.}
+
+  %% +-----------------------------------+
+  \ccHeading{Other free functions for \ccc{Uncertain<bool>}}
+
+  \ccFunction{bool certainly(Uncertain<bool> u);}
+  {returns \ccc{true} iff \ccVar.\ccc{is_certain()}, and the \ccVar.\ccc{make_certain}()
+   returns \ccc{true}.}
+
+  \ccFunction{bool certainly(bool u);}
+  {returns \ccc{u}.}
+
+  \ccFunction{bool possibly(Uncertain<bool> u);}
+  {returns \ccc{true} iff \ccVar.\ccc{is_certain()} returns \ccc{false}, or if
+   \ccVar.\ccc{make_certain}() returns \ccc{true}.}
+
+  \ccFunction{bool possibly(bool u);}
+  {returns \ccc{u}.}
+
+
+\ccSeeAlso
+\ccc{CGAL::Interval_nt<bool>}
+
+\end{ccRefClass}
+
+\ccParDims
diff --git a/STL_Extension/doc_tex/STL_Extension_ref/intro.tex b/STL_Extension/doc_tex/STL_Extension_ref/intro.tex
index 8eac2e25f7b..1fc4283cc2e 100644
--- a/STL_Extension/doc_tex/STL_Extension_ref/intro.tex
+++ b/STL_Extension/doc_tex/STL_Extension_ref/intro.tex
@@ -113,5 +113,6 @@
 \ccRefIdfierPage{CGAL::Tag_true}\\
 \ccRefIdfierPage{CGAL::Tag_false}\\
 \ccRefIdfierPage{CGAL::Null_tag}\\
-\ccRefIdfierPage{CGAL::Null_functor}
+\ccRefIdfierPage{CGAL::Null_functor}\\
+\ccRefIdfierPage{CGAL::Uncertain<T>}
 %% EOF
diff --git a/STL_Extension/doc_tex/STL_Extension_ref/main.tex b/STL_Extension/doc_tex/STL_Extension_ref/main.tex
index 8a2b44ecc23..e8d4a123c64 100644
--- a/STL_Extension/doc_tex/STL_Extension_ref/main.tex
+++ b/STL_Extension/doc_tex/STL_Extension_ref/main.tex
@@ -32,6 +32,7 @@
 \input{STL_Extension_ref/multiset.tex}
 \input{STL_Extension_ref/Object.tex}
 \input{STL_Extension_ref/stl_extension.tex}
+\input{STL_Extension_ref/Uncertain.tex}
 
 %%\cgalColumnLayout
 
diff --git a/STL_Extension/include/CGAL/Uncertain.h b/STL_Extension/include/CGAL/Uncertain.h
index 99133df0c31..b9080cfe192 100644
--- a/STL_Extension/include/CGAL/Uncertain.h
+++ b/STL_Extension/include/CGAL/Uncertain.h
@@ -21,6 +21,7 @@
 #define CGAL_UNCERTAIN_H
 
 #include <CGAL/config.h>
+#include <CGAL/assertions.h>
 #include <CGAL/enum.h>
 #include <stdexcept>
 
@@ -76,8 +77,8 @@ public:
 };
 
 
-// Encodes an interval [min;max] of values of type T.
-// The primary template is supposed to work for enums and bool.
+// Encodes a range [inf,sup] of values of type T.
+// T can be enums or bool.
 
 template < typename T >
 class Uncertain
@@ -88,10 +89,10 @@ class Uncertain
 
 public:
 
-  typedef CGAL::Uncertain_conversion_exception  Uncertain_conversion_exception;
-
   typedef T  value_type;
 
+  typedef CGAL::Uncertain_conversion_exception  Uncertain_conversion_exception;
+
   Uncertain()
     : _i(CGALi::Minmax_traits<T>::min),
       _s(CGALi::Minmax_traits<T>::max) {}
@@ -100,12 +101,12 @@ public:
     : _i(t), _s(t) {}
 
   Uncertain(T i, T s)
-    : _i(i), _s(s) {}
+    : _i(i), _s(s) { CGAL_precondition(i <= s); }
 
   Uncertain& operator=(T t)
   {
     _i = _s = t;
-    return this;
+    return *this;
   }
 
   T inf() const { return _i; }
@@ -173,13 +174,6 @@ T sup(Uncertain<T> i)
 }
 
 
-// possibly() declarations (needed as used in the assertions)
-// ----------------------------------------------------------
-
-inline bool possibly(bool b);
-inline bool possibly(Uncertain<bool> c);
-
-
 // Basic functions
 // ---------------
 
diff --git a/STL_Extension/include/CGAL/assertions.h b/STL_Extension/include/CGAL/assertions.h
index b7a268afb2a..ac1335ad11f 100644
--- a/STL_Extension/include/CGAL/assertions.h
+++ b/STL_Extension/include/CGAL/assertions.h
@@ -47,6 +47,13 @@ void postcondition_fail  ( const char*, const char*, int, const char* = "");
 void warning_fail( const char*, const char*, int, const char* = "");
 
 
+// The following declarations are needed first, before assertions are used.
+// ---------------------------------------------------------------------
+template < typename T > class Uncertain;
+inline bool possibly(bool b);
+inline bool possibly(Uncertain<bool> c);
+
+
 // macro definitions
 // =================
 // assertions
diff --git a/STL_Extension/test/STL_Extension/test_Uncertain.cpp b/STL_Extension/test/STL_Extension/test_Uncertain.cpp
new file mode 100644
index 00000000000..32f9e7692ad
--- /dev/null
+++ b/STL_Extension/test/STL_Extension/test_Uncertain.cpp
@@ -0,0 +1,268 @@
+// Sylvain Pion, 2008.
+
+#include <CGAL/Uncertain.h>
+#include <cassert>
+#include <iostream>
+#include <CGAL/enum.h>
+#include <CGAL/exceptions.h>
+
+// "unused variable" warning killer
+template <typename T>
+void use(T) {}
+
+// generic test , for both enums and bool
+template < typename T >
+void test()
+{
+	std::cout << "Testing Uncertain<T>\n";
+
+	typedef CGAL::Uncertain<T> U;
+
+	// Nested types.
+	typename U::value_type t = T();
+	typename U::Uncertain_conversion_exception e = CGAL::Uncertain_conversion_exception("");
+
+	// Constructors, assignment.
+	const T t0 = static_cast<T>(0);
+	const U u;
+	const U v = t0;
+	U w = U (T(), T());
+	U v2 = u;
+	w = (w = t0);
+
+	// Various functions
+	t = u.inf();
+	t = u.sup();
+
+	assert(! u.is_certain());
+	assert(  v.is_certain());
+	assert(t0 == v.make_certain());
+
+	T conv = v;
+	assert(conv == t0);
+
+	U indet = U::indeterminate();
+
+	t = CGAL::inf(u);
+	t = CGAL::sup(u);
+
+	assert(! CGAL::is_certain(u));
+	assert(  CGAL::is_certain(v));
+	assert(  CGAL::is_certain(t0));
+	assert(  CGAL::is_indeterminate(u));
+	assert(! CGAL::is_indeterminate(v));
+	assert(! CGAL::is_indeterminate(t0));
+	assert(t0 == CGAL::get_certain(t0));
+	assert(t0 == CGAL::get_certain(v));
+	assert(t0 == CGAL::make_certain(t0));
+	assert(t0 == CGAL::make_certain(v));
+
+	// Exceptions
+	bool ok = false;
+	try { CGAL::get_certain(u); }
+	catch (CGAL::Assertion_exception) { ok = true; }
+	assert(ok);
+	ok = false;
+	try { CGAL::make_certain(u); T t = u; use(t); }
+	catch (CGAL::Uncertain_conversion_exception) { ok = true; }
+	assert(ok);
+
+	U u2 = CGAL::make_uncertain(u);
+	U u3 = CGAL::make_uncertain(T());
+
+	// Operators
+	assert( v == v );
+	assert( v == t0 );
+	assert( t0 == v );
+	assert( ! (v != v) );
+	assert( ! (v != t0) );
+	assert( ! (t0 != v) );
+
+	use(t);
+}
+
+
+// test only for enums
+template < typename T >
+void test_enum()
+{
+	std::cout << "Testing Uncertain<enum>\n";
+
+	test<T>(); // generic test
+
+	typedef CGAL::Uncertain<T> U;
+
+	T n = static_cast<T>(-1);
+	T z = static_cast<T>(0);
+	T p = static_cast<T>(1);
+
+	U indet;
+
+	// <, <=, >, >=
+	assert(U(n) < U(z));
+	assert(U(z) < U(p));
+	assert(U(n) < z);
+	assert(n < U(z));
+	assert(! CGAL::is_certain(indet < z));
+
+	assert(U(n) <= U(z));
+	assert(U(z) <= U(p));
+	assert(U(z) <= U(z));
+	assert(U(n) <= z);
+	assert(U(n) <= n);
+	assert(n <= U(z));
+	assert(z <= U(z));
+	assert(! CGAL::is_certain(indet <= z));
+	assert(! CGAL::is_certain(indet <= indet));
+
+	assert(U(z) > U(n));
+	assert(U(p) > U(z));
+	assert(U(z) > n);
+	assert(z > U(n));
+	assert(! CGAL::is_certain(z > indet));
+
+	assert(U(z) >= U(n));
+	assert(U(p) >= U(z));
+	assert(U(z) >= U(z));
+	assert(U(z) >= n);
+	assert(U(n) >= n);
+	assert(z >= U(n));
+	assert(z >= U(z));
+	assert(! CGAL::is_certain(z >= indet));
+	assert(! CGAL::is_certain(indet >= indet));
+
+	assert(-p == n);
+	assert(-n == p);
+	assert(-z == z);
+}
+
+// test only for enums with multiplication operator
+template < typename T >
+void test_mult_enum()
+{
+	std::cout << "Testing Uncertain<enum with operator*>\n";
+
+	test_enum<T>(); // generic enum test.
+
+	typedef CGAL::Uncertain<T> U;
+
+	T n = static_cast<T>(-1);
+	T z = static_cast<T>(0);
+	T p = static_cast<T>(1);
+
+	U indet;
+
+	assert(n*z == z);
+	assert(z*n == z);
+	assert(z*z == z);
+	assert(p*z == z);
+	assert(z*p == z);
+	assert(n*n == p);
+	assert(p*p == p);
+	assert(n*p == n);
+	assert(indet*z == z);
+	assert(z*indet == z);
+	assert(CGAL::is_indeterminate(p*indet));
+	assert(CGAL::is_indeterminate(n*indet));
+}
+
+
+// test only for bool
+void test_bool()
+{
+	std::cout << "Testing Uncertain<bool>\n";
+
+	test<bool>(); // generic test
+
+	typedef CGAL::Uncertain<bool> U;
+
+	U utrue = true;
+	U ufalse = false;
+	U indet;
+
+	assert(utrue);
+	assert(!ufalse);
+	assert(ufalse == !utrue);
+	assert(utrue != !utrue);
+	assert(false == !utrue);
+	assert(true != !utrue);
+
+	assert(utrue | utrue);
+	assert(utrue | ufalse);
+	assert(ufalse | utrue);
+	assert(! (ufalse | ufalse));
+	assert(utrue | true);
+	assert(utrue | false);
+	assert(ufalse | true);
+	assert(! (ufalse | false));
+	assert(true | utrue);
+	assert(true | ufalse);
+	assert(false | utrue);
+	assert(! (false | ufalse));
+
+	assert(utrue & utrue);
+	assert(! (utrue & ufalse));
+	assert(! (ufalse & utrue));
+	assert(! (ufalse & ufalse));
+	assert(utrue & true);
+	assert(! (utrue & false));
+	assert(! (ufalse & true));
+	assert(! (ufalse & false));
+	assert(true & utrue);
+	assert(! (true & ufalse));
+	assert(! (false & utrue));
+	assert(! (false & ufalse));
+
+	// Test exceptions
+	bool ok = false;
+	try { bool b = indet; use(b); }
+	catch (CGAL::Uncertain_conversion_exception) { ok = true; }
+	assert(ok);
+	// The following must throw.
+	ok = false;
+	try { U u = indet && utrue; u = indet || ufalse; }
+	catch (CGAL::Uncertain_conversion_exception) { ok = true; }
+	assert(ok);
+	// The following must not throw.
+	try { bool b = utrue; b = ufalse; }
+	catch (CGAL::Uncertain_conversion_exception) { assert(false); }
+
+	// certainly, possibly
+	assert(CGAL::certainly(true));
+	assert(CGAL::certainly(utrue));
+	assert(!CGAL::certainly(indet));
+	assert(!CGAL::certainly(false));
+	assert(!CGAL::certainly(ufalse));
+
+	assert(CGAL::possibly(true));
+	assert(CGAL::possibly(utrue));
+	assert(CGAL::possibly(indet));
+	assert(!CGAL::possibly(false));
+	assert(!CGAL::possibly(ufalse));
+}
+
+void test_enum_cast()
+{
+	typedef CGAL::Uncertain<CGAL::Sign> Us;
+	typedef CGAL::Uncertain<CGAL::Bounded_side> Ub;
+	typedef CGAL::Uncertain<CGAL::Angle> Ua;
+
+	Us s;
+	Ub b = CGAL::enum_cast<CGAL::Bounded_side>(s);
+	Ua a = CGAL::enum_cast<CGAL::Angle>(s);
+	s = CGAL::enum_cast<CGAL::Sign>(b);
+}
+
+int main()
+{
+	test_bool();
+	test_mult_enum<CGAL::Sign>();
+	test_mult_enum<CGAL::Comparison_result>();
+	test_mult_enum<CGAL::Orientation>();
+	test_mult_enum<CGAL::Oriented_side>();
+	test_enum<CGAL::Bounded_side>();
+	test_enum<CGAL::Angle>();
+	test_enum_cast();
+
+	return 0;
+}