This article is from the FAQ, by with numerous contributions by others.
It is important to note that the fragment system INCLUDE mechanism is
radically different from e.g. the C compilers' #include facility. The C
#include mechanism is merely a means for textual composition, without any
semantical implication. The fragment system's INCLUDE mechanism is a
semantical, separate compilation facility, and at the same time it describes
parts of the dependency relations between the program parts.