'Note that the algorithm for shift is the SAME for arc-standard and arc-eager
:param configuration: is the current configuration
:return : A new configuration or -1 if the pre-condition is not satisfied'

def shift(self, conf):

    if (len(conf.buffer) <= 0):
        return (-1)
    idx_wi = conf.buffer.pop(0)
    conf.stack.append(idx_wi)
