docs: function forcelink(src, dest, options, callback) {
      if (!options || !options.cache) throw new Error('options.cache not defined!');
      if (!callback) throw new Error('callback not defined!');
      // uses relative path if linking to cache dir
      if (path.relative) {
          src = path.relative(options.cache, dest).slice(0, 2) !== '..' ? path.relative(path.dirname(dest), src) : src;
      }
      fs.lstat(dest, function(err, stat) {
          // Error.
          if (err && err.code !== 'ENOENT') {
              return callback(err);
          }
  
          // Path does not exist. Symlink.
          if (err && err.code === 'ENOENT') {
              if (env == 'development') console.error("[millstone] linking '" + dest + "' -> '"  + src + "'");
              return fs.symlink(src, dest, callback);
          }
  
          // Path exists and is not a symlink. Do nothing.
          if (!stat.isSymbolicLink()) {
              if (env == 'development') console.error("[millstone] skipping re-linking '" + src + "' because '"  + dest + " is already an existing file");
              return callback();
          }
  
          // Path exists and is a symlink. Check existing link path and update if necessary.
          // NOTE : broken symlinks will pass through this step
          fs.readlink(dest, function(err, old) {
              if (err) return callback(err);
              if (old === src) return callback();
              fs.unlink(dest, function(err) {
                  if (err) return callback(err);
                  if (env == 'development') console.error("[millstone] re-linking '" + dest + "' -> '"  + src + "'");
                  fs.symlink(src, dest, callback);
              });
          });
      });
  }