I would like to look at the code that implements
FillingTransform. Does Mathematica provide any way to do this?
In general, @Jacob Akkerboom is correct: one cannot usually view the source code for arbitrary Mathematica functions. In this particular case, however, we find an exception.
we observe a call to
reveals a number of checks on the arguments and setup for a call to
then the complete implementation becomes visible. Although I won't reproduce that here for obvious reasons, the basic idea is a combination of