Accessor of the expander-column slot of the gtk-tree-view class.
The gtk-tree-view-expander-column slot access function returns the column that is the current expander column. This column has the expander arrow drawn next to it.
The (setf gtk-tree-view-expander-column) slot access function sets the column to draw the expander arrow at. It must be in the tree view. If column is nil, then the expander arrow is always at the first visible column.
If you do not want expander arrow to appear in your tree, set the expander column to a hidden column.