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    "# Draw a repeatedly expanding/contracting circle\n",
    "# pip3 install drawsvg\n",
    "import drawSvg as draw\n",
    "\n",
    "# Draw a frame of the animation\n",
    "def draw_frame(r):\n",
    "    d = draw.Drawing(250, 250, origin='center')\n",
    "    d.append(draw.Circle(0, 0, r, fill='blue'))\n",
    "    return d\n",
    "               \n",
    "# run animation\n",
    "c = 0 # loop counter\n",
    "r = 0 # starting radius\n",
    "R = 100 # ending radius\n",
    "sign = 1 # expansion (1) or contraction (-1)\n",
    "\n",
    "with draw.animate_jupyter(draw_frame, delay=0.01) as anim:\n",
    "    while c < 4:\n",
    "        anim.draw_frame(r)\n",
    "        if sign == 1:\n",
    "            r = r + 1 # expand\n",
    "        else:\n",
    "            r = r - 1 # contract\n",
    "            \n",
    "        if r > R or r < 0:\n",
    "            sign = -1 * sign # flip sign of expansion/contraction\n",
    "            c = c + 1 # loop counter"
   ]
  }
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