code: public PointExpression<Point> pointOnSurface() {
          if (pointOnSurface == null) {
              pointOnSurface = GeometryExpressions.pointOperation(SpatialOps.POINT_ON_SURFACE, mixin);
          }
          return pointOnSurface;
      }
docs: A Point guaranteed to be on this MultiSurface.
  
  @return point on surface