Only the recursions are given for each step, since the certificates are extremely long.

f(n,r,s) := binomial(n,r)*binomial(n,s)*binomial(n+r,r)*binomial(n+s,s)*binomial(2*n-r-s,n)

Step 1.

l := 2

Step 2.

Rec[2,0] := N+(n+1+r)*(n+1+s)*(-2*n+r+s-1)*(-2*n-2+r+s)/(n+1)/(-n-1+r)/(-n-1+s)/(-n-1+r+s)

Rec[2,1] := R+(n+1+r)*(-n+r)*(-n+r+s)/(r+1)^2/(-2*n+r+s)

REC[2] := S+(n+1+s)*(-n+s)*(-n+r+s)/(s+1)^2/(-2*n+r+s)

Step 3.

REC[1] := -(n+3+r)*(n+2+r)*(-n+r+1)*(r^2-4*n*r+r+4*n^2+1-n)/(r+3)^2/(r-3*n+1)/(r+2)^2*R+(n+3+r)*(r^2+2*r-2*n*r-4*n+1-2*n^2)/(r+3)^2/(r-3*n+1)*R^2+R^3-(n+2+r)*(n+1+r)*(-n+r+1)*(n+3+r)*(-n+r)^3/(r+2)^2/(...
REC[1] := -(n+3+r)*(n+2+r)*(-n+r+1)*(r^2-4*n*r+r+4*n^2+1-n)/(r+3)^2/(r-3*n+1)/(r+2)^2*R+(n+3+r)*(r^2+2*r-2*n*r-4*n+1-2*n^2)/(r+3)^2/(r-3*n+1)*R^2+R^3-(n+2+r)*(n+1+r)*(-n+r+1)*(n+3+r)*(-n+r)^3/(r+2)^2/(...

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Step 5.

Rec[1,0] := N+1/2*(r+1)^2*(4*r^4-26*n*r^3-13*r^3+67*n^2*r^2+71*n*r^2+14*r^2-84*n^3*r-141*n^2*r-62*n*r-5*r+45*n^4+105*n^3+75*n^2+15*n)/n/(-n+r)/(-n-1+r)^3/(n+1)*R+1/2*(r+2)^2*(r+1)^2*(r-3*n)*(4*r^3-17*n...
Rec[1,0] := N+1/2*(r+1)^2*(4*r^4-26*n*r^3-13*r^3+67*n^2*r^2+71*n*r^2+14*r^2-84*n^3*r-141*n^2*r-62*n*r-5*r+45*n^4+105*n^3+75*n^2+15*n)/n/(-n+r)/(-n-1+r)^3/(n+1)*R+1/2*(r+2)^2*(r+1)^2*(r-3*n)*(4*r^3-17*n...
Rec[1,0] := N+1/2*(r+1)^2*(4*r^4-26*n*r^3-13*r^3+67*n^2*r^2+71*n*r^2+14*r^2-84*n^3*r-141*n^2*r-62*n*r-5*r+45*n^4+105*n^3+75*n^2+15*n)/n/(-n+r)/(-n-1+r)^3/(n+1)*R+1/2*(r+2)^2*(r+1)^2*(r-3*n)*(4*r^3-17*n...
Rec[1,0] := N+1/2*(r+1)^2*(4*r^4-26*n*r^3-13*r^3+67*n^2*r^2+71*n*r^2+14*r^2-84*n^3*r-141*n^2*r-62*n*r-5*r+45*n^4+105*n^3+75*n^2+15*n)/n/(-n+r)/(-n-1+r)^3/(n+1)*R+1/2*(r+2)^2*(r+1)^2*(r-3*n)*(4*r^3-17*n...

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Step 6.

REC[0] := 1/2/(n+3)*(3364062*n^7+53792011*n^6+361561621*n^5+1323518594*n^4+2848918133*n^3+3606296853*n^2+2486801742*n+721236384)*(n+2)/(n+4)^3/(39117*n^4+292994*n^3+806493*n^2+965638*n+425694)*N+(20379...
REC[0] := 1/2/(n+3)*(3364062*n^7+53792011*n^6+361561621*n^5+1323518594*n^4+2848918133*n^3+3606296853*n^2+2486801742*n+721236384)*(n+2)/(n+4)^3/(39117*n^4+292994*n^3+806493*n^2+965638*n+425694)*N+(20379...
REC[0] := 1/2/(n+3)*(3364062*n^7+53792011*n^6+361561621*n^5+1323518594*n^4+2848918133*n^3+3606296853*n^2+2486801742*n+721236384)*(n+2)/(n+4)^3/(39117*n^4+292994*n^3+806493*n^2+965638*n+425694)*N+(20379...
REC[0] := 1/2/(n+3)*(3364062*n^7+53792011*n^6+361561621*n^5+1323518594*n^4+2848918133*n^3+3606296853*n^2+2486801742*n+721236384)*(n+2)/(n+4)^3/(39117*n^4+292994*n^3+806493*n^2+965638*n+425694)*N+(20379...
REC[0] := 1/2/(n+3)*(3364062*n^7+53792011*n^6+361561621*n^5+1323518594*n^4+2848918133*n^3+3606296853*n^2+2486801742*n+721236384)*(n+2)/(n+4)^3/(39117*n^4+292994*n^3+806493*n^2+965638*n+425694)*N+(20379...
REC[0] := 1/2/(n+3)*(3364062*n^7+53792011*n^6+361561621*n^5+1323518594*n^4+2848918133*n^3+3606296853*n^2+2486801742*n+721236384)*(n+2)/(n+4)^3/(39117*n^4+292994*n^3+806493*n^2+965638*n+425694)*N+(20379...

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Step 7.

l := 1

Stop.