Headers
  Specific
    <Xm/PushB.h> - PushButton
    <Xm/MainW.h> - MainWindow
  Required
    <Xm/Xm.h> - utilities for creating widgets & groups of widgets:
      <X11/Intrinsic.h>,<11/Vendor.h>,<Xm/VirtKeys.h>,<Xm/VendorE.h>
      <Xm/VendorE.h>: <Xm/ExtObject.h>,<X11/Shell.h>,<X11/Vendor.h>
    <?Xt?> - creating & setting resources on widgets
    <?Xlib> - graphics rendering & environment input
Initialization
  topwidgetname = XtVaAppInitialize(
    &app,  #declared with 'XtAppContext app;'
    " ",  #class name of application  *CONVENTION*: filename=Classname (caps)
    cmd-line arguments, ... ,  #command-line arguments to the application
    argc, argv,  #cmd-line argument variables
    default_resources,  #p.24
    resource_value_pairs_NULL_terminated );
Method
  1) compiler line: {alias com "cc -o \!* \!*.c -lXm -lXt -lX11 -lm"}
  2) header files
  3) main(argc, argv) int argc; char *argv[];
  4) Widgets, XtAppContext, Functions, variables
  5) Initialize
  ...
  6) free used strings
  7) callbacks(?)
  8) Realize the Widgets
  9) Main Loop Initialization
Resources
  XtVaSetValues() - change values at run-time
Widget Classes
  Primitive
  Label
  PushButton
  DrawButton
  ToggleButton
  CascadeButton
  ArrowButton
  List
  Scrollbar
  Separator
Manager Widget Classes
  DrawingArea
  ScrolledWindow
  MainWindow
  RowColumn
  Frame
  PanedWindow
  BulletinBoard
  Form
  Scale  
Dialogs
  ErrorDialog (do-not-enter symbol)
  InformationDialog ((i)nformation)
  QuestionDialog (?yes,no,cancel)
  WarningDialog (!warning)
  WorkingDialog (hourglass waiting)