class List:
    def __init__(self, elem=None):
        """
        pre:  -
        post: initialize a new List with `elem` as first element,
              if `elem` is None, this is interpreted as an empty List
        """
        self.first_elem = elem   # Element in first position
        if elem is not None:
            self.tail_list = List()  # Tail list of the current List
        else:
            self.tail_list = None

    def head(self):
        """
        pre: current List is not empty
        post: return the first element of the current List
        """
        if self.is_empty():
            raise RuntimeError('Cannot get first element from an empty List')
        return self.first_elem

    def tail(self):
        """
        pre: current List is not empty
        post: return the tail List of the current List
        """
        if self.is_empty():
            raise RuntimeError('Cannot get tail List from an empty List')
        return self.tail_list

    def is_empty(self):
        """
        pre: -
        post: return True if the List is empty, False otherwise
        """
        return self.first_elem is None

    def concat(self, elem):
        """
        pre: `elem` is not None
        post: return a new List with `elem` in first position
              and the current List as tail
        """
        if elem is None:
            raise RuntimeError('Cannot concat a None element')
        newlist = List(elem=elem)
        newlist.tail_list = self
        return newlist

    def __str__(self):
        """
        pre: -
        post: return the List content as a string
        """
        if self.is_empty():
            return '[]'
        return '['+ str(self.head()) + str(self.tail()) +']'