code: public long allocAppend(long cluster)
              throws IOException {
          
          testCluster(cluster);
          
          while (!isEofCluster(entries[(int) cluster])) {
              cluster = entries[(int) cluster];
          }
          
          long newCluster = allocNew();
          entries[(int) cluster] = newCluster;
  
          return newCluster;
      }
docs: Allocate a cluster to append to a new file
  
  @param cluster a cluster from a chain where the new cluster should be
  appended
  @return long the newly allocated and appended cluster number
  @throws IOException if there are no free clusters