@Override
  public long decr(String key, int by, long def) {
    return mutateWithDefault(Mutator.decr, key, by, def, 0);
  }
Decrement the given counter, returning the new value.

@param key the key
@param by the amount to decrement
@param def the default value (if the counter does not exist)
@return the new value, or -1 if we were unable to decrement or add
@throws OperationTimeoutException if the global operation timeout is
          exceeded
@throws IllegalStateException in the rare circumstance where queue is too
          full to accept any more requests
src/main/java/net/spy/memcached/MemcachedClient.java