docs: def flatten(L):
      """
      Flattens a list.
  
      Example:
  
      >>> flatten([a,b,[c,d,[e,f]]])
      [a,b,c,d,e,f]
      """
      if not isinstance(L, list):
          return [L]
  
      if L == []:
          return L
  
      return flatten(L[0]) + flatten(L[1:])