... which allow setting/getting the "underlying operation" node of
a graph node.  For example, GimpDrawableFilter constructs a complex
graph around a given operation node, which would be the underlying
operation of the graph.  This allows querying the properties of the
underlying operation, given only the graph.

In recursive cases, gimp_gegl_node_get_underlying_operation()
returns the most-nested underlying operation; when no underlying
operation has been set, gimp_gegl_node_get_underlying_operation()
returns the input node.