input: Write a python function to fetch and store articles from PubMed's COVID-19 search page into a MongoDB collection. 
  assert fetch_and_store_articles() is None
output: def fetch_and_store_articles():
      import requests
      from pymongo import MongoClient
      import json
  
      # Define the PubMed COVID-19 search URL
      url = "https://api.ncbi.nlm.nih.gov/lit/ctxp/v1/pubmed/?term=COVID-19"
  
      # Make a request to the PubMed API
      response = requests.get(url)
      
      if response.status_code == 200:
          articles = response.json()
          
          # Connect to the MongoDB server
          client = MongoClient('localhost', 27017)
          
          # Select or create the database
          db = client['pubmed_db']
          
          # Select or create the collection
          collection = db['covid19_articles']
          
          # Insert articles into the collection
          if isinstance(articles, list):
              collection.insert_many(articles)
          else:
              collection.insert_one(articles)
      else:
          print("Failed to fetch articles from PubMed")