PostGIS  2.1.10dev-r@@SVN_REVISION@@
PG_FUNCTION_INFO_V1 ( LWGEOM_interiorringn_polygon  )

InteriorRingN(GEOMETRY) – find the first polygon in GEOMETRY, Index is 1-based.

Returns
its Nth interior ring (as a linestring). Return NULL if there is no POLYGON(..) in (first level of) GEOMETRY.