      "        if self.first is None and self.last is None:\n",
      "            return None\n",
      "        data = self.first.data\n",
      "        # Remove only element from a one element list\n",
      "        if self.first == self.last:\n",
      "            self.first = None\n",
      "            self.last = None\n",
      "        else:\n",
      "            self.first = self.first.next\n",
      "        return data"
     ]
    },
 
</s> remove     "        data = self.first.data\n",
 </s> add     "        data = self.head.data\n", </s> remove     "        if self.first is None and self.last is None:\n",
 </s> add     "        if self.head is None and self.tail is None:\n", </s> remove     "            self.first = self.first.next\n",
 </s> add     "            self.head = self.head.next\n", </s> remove         if self.first == self.last:
            self.first = None
            self.last = None
 </s> add         if self.head == self.tail:
            self.head = None
            self.tail = None </s> remove         data = self.first.data
 </s> add         data = self.head.data
    "        if self.head == self.tail:\n",
    "            self.head = None\n",
    "            self.tail = None\n",
      "        if self.first is None and self.last is None:\n",
      "            return None\n",
      "        data = self.first.data\n",
      "        # Remove only element from a one element list\n",
    "        if self.head == self.tail:\n",
    "            self.head = None\n",
    "            self.tail = None\n",
    "        if self.head == self.tail:\n",
    "            self.head = None\n",
    "            self.tail = None\n",
    "        if self.head == self.tail:\n",
    "            self.head = None\n",
    "            self.tail = None\n",
      "        else:\n",
      "            self.first = self.first.next\n",
      "        return data"
     ]
    },
keep keep keep keep replace replace replace keep keep keep keep keep
 Polish queue challenge and solution (#70)

Update constraints, algorithm discussion, and code. 
stacks_queues/queue_list/queue_list_solution.ipynb