android/guava/src/com/google/common/collect/StandardTable.java
@Override
  public Set<C> columnKeySet() {
    Set<C> result = columnKeySet;
    return (result == null) ? columnKeySet = new ColumnKeySet() : result;
  }
@Override
  public Set<C> columnKeySet() {
    Set<C> result = columnKeySet;
    return (result == null) ? columnKeySet = new ColumnKeySet() : result;
  }
{@inheritDoc}

<p>The returned set has an iterator that does not support {@code remove()}.

<p>The set's iterator traverses the columns of the first row, the columns of the second row,
etc., skipping any columns that have appeared previously.