public BooleanExpression isRing() {
        if (ring == null) {
            ring = Expressions.booleanOperation(SpatialOps.IS_RING, mixin);
        }
        return ring;
    }
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
querydsl-spatial/src/main/java/com/querydsl/spatial/jts/JTSCurveExpression.java