The command tt(INCLUDEFILE) takes one argument, a filename. The file is
included. 

The tt(yodl) program supplies, when necessary, an extension to the filename.
The supplied extension is tt(.yo), unless defined otherwise in the compilation
of the program. Furthermore, tt(yodl) prefixes the file with the system-wide
include directory if necessary. This directory is normally
tt(/usr/local/lib/yodl), unless overruled by the command line flag tt(-I) or
defined otherwise in the compilation.

E.g., if you install a macro package in a file tt(latex.yo) in the directory
tt(/usr/local/lib/yodl), then the command

verb(INCLUDEFILE(latex))

will include it.
