docs: function localize(url, options, callback) {
      existsAsync(options.filepath, function(exists) {
          if (exists) {
              var re_download = false;
              // unideal workaround for frequently corrupt/partially downloaded zips
              // https://github.com/mapbox/millstone/issues/85
              if (path.extname(options.filepath) == '.zip') {
                  try {
                    var zf = new zipfile.ZipFile(options.filepath);
                    if (zf.names.length < 1) {
                        throw new Error("could not find any valid data in zip archive: '" + options.filepath + "'");
                    }
                  } catch (e) {
                      if (env == 'development') console.error('[millstone] could not open zip archive: "' + options.filepath + '" attempting to re-download from "'+url+"'");
                      re_download = true;
                  }
              }
              if (!re_download) {
                  return callback(null, options.filepath);
              }
          }
          var dir_path = path.dirname(options.filepath);
          mkdirp(dir_path, 0755, function(err) {
              if (err && err.code !== 'EEXIST') {
                  if (env == 'development') console.error('[millstone] could not create directory: ' + dir_path);
                  callback(err);
              } else {
                  download(url, options, function(err, filepath) {
                      if (err) return callback(err);
                      callback(null, filepath);
                  });
              }
          });
      });
  }