Multigraphs with a given degree sequence

Haskell, 57 bytes

f(h:t)=sum[f l|l<-mapM(\n->[0..n])t,sum t-sum l==h]
f _=1

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Haskell, 127 bytes

e=[]:e
h a b=mapM id$b<$a
f a=length[0|x<-h a$h a[0..maximum a],x==foldr(zipWith(:))e x,all(<1)$zipWith(!!)x[0..],map sum x==a]

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Very very inefficient (superexponential), but I believe it should work in theory. Exceeds Tio's 60 second limit with [3, 3, 1, 1] as input, but works with [1, 1, 1, 1].

Considers the adjacency matrices of the multigraphs.

h a$h a[0..maximum a] is a list of all square matrices with entries between 0 and maximum a, with size length a.

x==foldr(zipWith(:))e x checks if a matrix is symmetric.

all(<1)$zipWith(!!)x[0..] checks that the diagonal entries are zero, since there are no loops.

map sum x==a checks that each vertex has the proper degree.

Haskell, 75 bytes

f(h:t)=sum$f<$>h%t
f[]=1
r%(h:t)=[h-i:x|i<-[0..r],x<-(r-i)%t]
r%_=[[]|r==0]

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It's faster (and easier to understand) when i<-[0..r] is replaced by i<-[0..min r h]. Also, it's faster to give the degrees (whose order obviously doesn't matter) in increasing order.