If I have the following in my interface.ccl,
USES INCLUDE: vectors.h
but no thorn in my thornlist provides this header file, there is no error at compile time. Further, if I #include this file in my source file, an empty file is included, which means that again I don't get an error. The first indication that something is wrong is that the contents of the header file are not available, which makes debugging the problem with the thornlist very confusing.
I propose that the CST should emit a fatal error if one of the thorns tries to use a header file which does not exist.