Coding style.
Upgrade warning into error. Surround the names with ticks.
Test overloaded calls partially outside the custom derivatives namespace.
Inform by setting m_DerivedFD that we are in recursive differentiation.
Style.
If we are recursively deriving add the found candidate in the LookupResult.
Adjust the test to the new behavior
Implement support for IfStmts, makes test NonContinuous.C go further.
Turn a non working case into working. Remove dups.
No we rely on the argument number to denote the independent variable.

We cannot get rid of the variadic templates, because we need to get the function
signature somehow.
Blank spaces and indents
Use Sema instead of creating raw AST nodes.
Handle the case where unary operator is involved. Eg. return -x*x.