The interface between HIGZ and the X Window System (X11) is done via a set
of C routines callable from a Fortran program. This set of routine provide
a low level interface to X11. It allows to write X11 programs with a small
number of simple routines and it does not require the knowledge of the
complete X11 library (Xlib). This set of interface routines are described
in this appendix. The "normal" HIGZ user does not need to read this
chapter, which is useful only to write X11 drivers in Fortran.