'Initialize the multi-word tokenizer with a list of expressions and a
separator
:type mwes: list(list(str))
:param mwes: A sequence of multi-word expressions to be merged, where
each MWE is a sequence of strings.
:type separator: str
:param separator: String that should be inserted between words in a multi-word
expression token. (Default is \'_\')'

def __init__(self, mwes=None, separator='_'):

    if (not mwes):
        mwes = []
    self._mwes = Trie(mwes)
    self._separator = separator
