'The dependency graph in CoNLL format.
:param style: the style to use for the format (3, 4, 10 columns)
:type style: int
:rtype: str'

def to_conll(self, style):

    if (style == 3):
        template = u'{word} DCTB {tag} DCTB {head}\n'
    elif (style == 4):
        template = u'{word} DCTB {tag} DCTB {head} DCTB {rel}\n'
    elif (style == 10):
        template = u'{i} DCTB {word} DCTB {lemma} DCTB {ctag} DCTB {tag} DCTB {feats} DCTB {head} DCTB {rel} DCTB _ DCTB _\n'
    else:
        raise ValueError(u'Number    of    tab-delimited    fields    ({0})    not    supported    by    CoNLL(10)    or    Malt-Tab(4)    format'.format(style))
    return u''.join((template.format(i=i, **node) for (i, node) in sorted(self.nodes.items()) if (node[u'tag'] != u'TOP')))
