'Add a multi-word expression to the lexicon (stored as a word trie)
We use ``util.Trie`` to represent the trie. Its form is a dict of dicts.
The key True marks the end of a valid MWE.
:param mwe: The multi-word expression we\'re adding into the word trie
:type mwe: tuple(str) or list(str)
:Example:
>>> tokenizer = MWETokenizer()
>>> tokenizer.add_mwe((\'a\', \'b\'))
>>> tokenizer.add_mwe((\'a\', \'b\', \'c\'))
>>> tokenizer.add_mwe((\'a\', \'x\'))
>>> expected = {\'a\': {\'x\': {True: None}, \'b\': {True: None, \'c\': {True: None}}}}
>>> tokenizer._mwes.as_dict() == expected
True'

def add_mwe(self, mwe):

    self._mwes.insert(mwe)
