d1=1:3
d2=4:6
X=matrix(1:9,3,3)
all(DXD(d1, X, d2) == diag(d1) %*% X %*% diag(d2))
S=matrix(c(1,2,3,2,4,5,3,5,8),3,3)
X=matrix(1:9,3,3)
all( symprod(S, X) == S %*% X )
x=1:3
y=4:6
S=matrix(c(1,2,3,2,4,5,3,5,8),3,3)
txSy(x, S, y) == drop(t(x)%*%S%*%y)
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