code: public BooleanExpression isRing() {
          if (ring == null) {
              ring = Expressions.booleanOperation(SpatialOps.IS_RING, mixin);
          }
          return ring;
      }
docs: Returns 1 (TRUE) if this Curve is closed [StartPoint ( ) = EndPoint ( )] and this Curve is
  simple (does not pass through the same Point more than once).
  
  @return ring