In order to be able to use various data structures we must include header files.
NOTE: The X system is very large and there are many header files. Motif header files are found in #include<Xm/...>, The Xt and Xlib header files in #include<X11/...>. Xt Intrinsic definitions are in <X11/Intrinsics.h>
Every Motif widget has its own header file so we have to include the <Xm/PushB.h> file for the push button widget in push.c
We do not have to explicitly include the Xt header file as <Xm/Xm.h> does this automatically. Every Motif program will include <Xm/Xm.h> - the general header for the motif library.