####     ####  ####   Order 1   ####     ####     ####  

SS[1]:=[ 1/ (x*z + y*z - x - y - z + 1), (4*x-1)*Dx+2]:    
	# Central binomial coefficients: C(2*n,n) = (2*n)!/(n!)^2. 
	# http://oeis.org/A000984
	# 1/(4*x-1)^(1/2)

SS[2]:=[1/ (x*y*z - x*y + x*z + y*z - x - y - z + 1), (x^2-6*x+1)*Dx+x-3]:   
	# Diag (1 / ((-x*y - x - y + 1) * (t*z - t - z + 1)), 6):  
	# Central Delannoy numbers: a(n) = Sum_{k=0..n} C(n,k)*C(n+k,k).
	# http://oeis.org/A001850
    	# 1/(x^2-6*x+1)^(1/2)