code: @Override
    public Map<R, V> column(C columnKey) {
      checkNotNull(columnKey);
      Integer columnIndex = columnKeyToIndex.get(columnKey);
      return (columnIndex == null) ? ImmutableMap.<R, V>of() : new Column(columnIndex);
    }
docs: Returns a view of all mappings that have the given column key. If the column key isn't in
  {@link #columnKeySet()}, an empty immutable map is returned.
  
  <p>Otherwise, for each row key in {@link #rowKeySet()}, the returned map associates the row key
  with the corresponding value in the table. Changes to the returned map will update the
  underlying table, and vice versa.
  
  @param columnKey key of column to search for in the table
  @return the corresponding map from row keys to values