code: def clean_dataframe(df):
      """Clean up the dataframe.
  
      Args:
          df (pandas.DataFrame): Pandas dataframe.
  
      Returns:
          df (pandas.DataFrame): Cleaned pandas dataframe.
      """
  
      # Remove duplicated rows
      cols = ["cord_uid", "doi"]
      df = remove_duplicates(df, cols)
  
      # Remove rows without values in specified columns
      cols = ["cord_uid", "doi", "title", "license", "url"]
      df = remove_nan(df, cols)
  
      return df
  
file_path: recommenders/recommenders/datasets/covid_utils.py
imports: import numpy
  import pandas
  import requests
label_desc: Use image node_id 4 for calling a global function with example usage: clean_dataframe(df) and returns: df