'Constructs a Decoder.
Reads the text file describing the encoding and build the encoder.
The text file contains lines of the form:
<code>[,<code>]*\t<string>
Each line defines a mapping from a sequence of one or more integer codes to
a corresponding utf-8 string.
Args:
filename:   Name of file defining the decoding sequences.'

def __init__(self, filename):

    self.decoder = []
    if filename:
        self._InitializeDecoder(filename)
