Add support for differing types in almost_equal

maths.hpp

@@ -22,6 +22,7 @@
 
 #include "annotations.hpp"
 
+#include <type_traits>
 
 template <typename T>
 T 
@@ -62,13 +63,24 @@ almost_equal (const T &a, const T &b)
 
 
 template <typename Ta, typename Tb>
-bool
+typename std::enable_if<
+    std::is_arithmetic<Ta>::value && std::is_arithmetic<Tb>::value,
+    bool
+>::type
 almost_equal (const Ta &a, const Tb &b) {
     return almost_equal <decltype(a + b)> (static_cast<decltype(a + b)>(a),
                                            static_cast<decltype(a + b)>(b));
 }
 
 
+template <typename Ta, typename Tb>
+typename std::enable_if<
+    !std::is_arithmetic<Ta>::value || !std::is_arithmetic<Tb>::value,
+    bool
+>::type
+almost_equal (const Ta &a, const Tb &b)
+    { return a == b; }
+
 const double PI = 3.141592653589793238462643;
 
 inline double