Singular

LIB "ncrat.lib";
ncInit(list("x", "y", "z"));
ncrat f = ncratFromString("x+y");
ncrep q = ncrepGet(f);
matrix A[2][2] = 1, 2, 3, 4;
matrix B[2][2] = 5, 6, 7, 8;
ncrep s = ncrepSubstitute(q, list(x, y), list(A, B));
print(q);
==> lvec=
==> 0,1,0,1
==> 
==> mat=
==> x, -1,0, 0, 
==> -1,0, 0, 0, 
==> 0, 0, y, -1,
==> 0, 0, -1,0  
==> 
==> rvec=
==> 0,
==> 1,
==> 0,
==> 1 
print(s);
==> lvec=
==> 0,0,1,0,0,0,1,0,
==> 0,0,0,1,0,0,0,1 
==> 
==> mat=
==> 1, 2, -1,0, 0, 0, 0, 0, 
==> 3, 4, 0, -1,0, 0, 0, 0, 
==> -1,0, 0, 0, 0, 0, 0, 0, 
==> 0, -1,0, 0, 0, 0, 0, 0, 
==> 0, 0, 0, 0, 5, 6, -1,0, 
==> 0, 0, 0, 0, 7, 8, 0, -1,
==> 0, 0, 0, 0, -1,0, 0, 0, 
==> 0, 0, 0, 0, 0, -1,0, 0  
==> 
==> rvec=
==> 0,0,
==> 0,0,
==> 1,0,
==> 0,1,
==> 0,0,
==> 0,0,
==> 1,0,
==> 0,1