'Inserts ``string`` into the Trie
:param string: String to insert into the trie
:type string: str
:Example:
>>> from nltk.collections import Trie
>>> trie = Trie(["ab"])
>>> trie
defaultdict(<class \'nltk.collections.Trie\'>, {\'a\': defaultdict(<class \'nltk.collections.Trie\'>, {\'b\': defaultdict(<class \'nltk.collections.Trie\'>, {True: None})})})'

def insert(self, string):

    if len(string):
        self[string[0]].insert(string[1:])
    else:
        self[Trie.LEAF] = None
