docs: function previous(version, migrations) {
    function cmp(a, b) { return parseInt(a, 10) > parseInt(b, 10); }
    var regex = RegExp('^(' + version + '):?.*$');
    var keys = _.keys(migrations).sort(cmp);
    for (var i = 0, found; i < keys.length; i++) {
      if (keys[i].match(regex)) {
        found = keys[i - 1];
        break;
      }
    }
    return found || '0000';
  }