code: function purgeExcess(dir, shrinkwrap, opts, cb) {
      if (typeof opts === 'function') {
          cb = opts;
          opts = {};
      }
  
      findExcess(dir, shrinkwrap, opts, null, function (err, excessFiles) {
          if (err) {
              // if no node_modules then nothing to purge
              if (err.code === 'ENOENT') {
                  return cb(null);
              }
              return cb(err);
          }
  
          var tasks = excessFiles.map(function (file) {
              var filePath = path.join(dir, file);
              console.log('removing', filePath);
              return rimraf.bind(null, filePath);
          });
  
          parallel(tasks, cb);
      });
  }
docs: /*  given the shrinkwrap & package.json, find all extra folders
  in top level node_modules directory and remove them
  
  Basically like `npm prune` except recursive