ldiv — polynomial matrix long division
[x]=ldiv(n,d,k)
x=ldiv(n,d,k) gives the k first coefficients of the long
division of n by d i.e. the Taylor expansion of the rational
matrix [nij(z)/dij(z)] near infinity.
Coefficients of expansion of nij/dij are stored in
x((i-1)*n+k,j) k=1:n