'Arguments:
layers: the layers to use for this Decoder
start_index: the index of the symbol to use as the start symbol
when generating a new sequence.  If None, defaults to
all 0 (not the same as index 0)'

def __init__(self, layers, name=None, start_index=None):

    super(Decoder, self).__init__(layers, name)
    self._recurrent = [l for l in self.layers if isinstance(l, Recurrent)]
    self.connections = None
    self.full_steps = None
    self.start_index = start_index
