code: def flatten(in_list):
      
      out_list = []
      for val in in_list:
          if isinstance(val, list):
              out_list.extend(val)
          else:
              out_list.append(val)
  
      return out_list
docs: given a list of values in_list, flatten returns the list obtained by 
         flattening the top-level elements of in_list.