begin_structure 20 ;		% Static structure 20 %
	  Non_uniform_bspline_surface
	  non_rational
	  2 2
	  2 2
		   70    35     0
		   70   -70     0
		  -35    35     0
		  -35   -70     0
      ; 

	  Non_uniform_bspline_surface
	  non_rational
	  2 2
	  2 2
		  -70   -35     0
		   35   -35     0
		  -70    70     0
		   35    70     0
      ; 
end_structure ;			% Structure 20  %