Paragraph 12.8/15 of the C++11 Standard:

The implicitly-defined copy/move constructor for a non-union class X
performs a memberwise copy/move of its bases and members

There is now a generic combined function and the combination action can be
defined by specializing a small internal class. The old Difference is now
implemented as a derived class.

-rename namespace Function to Functions
-rename class Functions to FunctionsProvider