def _render_digraph(pairs):
    """Renders paris of nodes to .dot-file with directional graph.
    """
    out = ['digraph container {']
    for frm, to in pairs:
        out.append('\t' + ('"{}:{}" -> '.format(*frm) if frm else '') +
            '"{}:{}";'.format(*to))
    out.append('}')
    return '\n'.join(out)
