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    "<center><h2>Classical Billiards on a Square Table</h2></center>\n",
    "\n",
    "    A billiard ball bounces around classically on a square table and is visuallizes with Vpython. \n",
    "    To be compared with the quantum version of this program \"SqBilliardQM\"."
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   "source": [
    "\"\"\" From \"COMPUTATIONAL PHYSICS\" & \"COMPUTER PROBLEMS in PHYSICS\"\n",
    "    by RH Landau, MJ Paez, and CC Bordeianu (deceased).\n",
    "    Copyright R Landau, Oregon State Unv, MJ Paez, Univ Antioquia, \n",
    "    C Bordeianu, Univ Bucharest, 2021. \n",
    "    Please respect copyright & acknowledge our work.\"\"\"\n",
    "\n",
    "# SqBillardCM.py: Animated classical billiards on square table\n",
    "\n",
    "% matplotlib notebook\n",
    "from vpython import *\n",
    "\n",
    "dt = 0.01;   Xo = -90.;  Yo =  -5.4;  v = vec(13.,13.1,0)\n",
    "r0 = r= vec(Xo,Yo,0); eps = 0.1;  Tmax = 500; tp = 0\n",
    "scene = display(width=500, height=500, range=120,\\\n",
    "                background=color.white, foreground=color.black)\n",
    "table = curve(pos=([vec(-100,-100,0),vec(100,-100,0),vec(100,100,0),\n",
    "                    vec(-100,100,0),vec(-100,-100,0)]))\n",
    "ball = sphere(pos=vec(Xo,Yo,0),color=color.red, radius=3,make_trail=True)\n",
    "\n",
    "for t in arange(0,Tmax,dt):\n",
    "    rate(5000) \n",
    "    tp = tp + dt   \n",
    "    r = r0 + v*tp\n",
    "    if(r.x>= 100 or r.x<=-100):         # Right and left walls\n",
    "       v = vector(-v.x,v.y,0)\n",
    "       r0 = vector(r.x,r.y,0)\n",
    "       tp = 0\n",
    "    if(r.y>= 100 or r.y<=-100):         # Top and bottom walls\n",
    "       v = vector(v.x,-v.y,0)\n",
    "       r0 = vector(r.x,r.y,0)\n",
    "       tp = 0\n",
    "    ball.pos=r           "
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