docs: def split_list_by(lst, key):
    """
    Splits a list by the callable *key* where a negative result will cause the
    item to be put in the first list and a positive into the second list.
    """
  
    first, second = [], []
    for item in lst:
      if key(item):
        second.append(item)
      else:
        first.append(item)
    return (first, second)