C++ in a nutshell: "If you are unable to work through the following, you may
not know the fundamentals of C++: 
A partially specialized templated class with an enable_if
for SFINAE, containing a variadic templated method which
takes a parameter pack, with a trailing return type which
is deduced based on an expression decltype, then using
perfect forwarding to call a policy method." (http://www.copperspice.com/pdf/ACCU-Modern-C++.pdf)