code: @Override
      public T remove(int x, int y, int z) {
          long key = Int21TripleHashed.key(x, y, z);
          return map.remove(key);
      }
docs: Removes the mapping for a key from this map if it is present (optional operation). More formally, if this map contains a mapping from key <code>k</code> to value <code>v</code> such that
  <code>key.equals(k)</code>, that mapping is removed. (The map can contain at most one such mapping.)<br> Returns the value to which this map previously associated the key, or <code>null</code>
  if the map contained no mapping for the key.<br> If this map permits null values, then a return value of <code>null</code> does not <i>necessarily</i> indicate that the map contained no
  mapping for the key; it's also possible that the map explicitly mapped the key to <code>null</code>.<br> The map will not contain a mapping for the specified key once the call returns.
  
  @param x an <code>int</code> value
  @param y an <code>int</code> value
  @param z an <code>int</code> value
  @return the previous <code>long</code> value associated with <code>key(x, y, z)</code>, or <code>null</code> if there was no mapping for key.