Singular

ring Rt = (0,s,t),(x,y),lp;  // s and t are  transcendal parameters

// define a 2x3 matrix with polynomial entries in x and y
// having rational coefficients in Q(s,t)
matrix m[2][3] = 2/t*x+1,1, xy, s/(t2-1),s/t*y,0;
m;
m[2,3] = -1;   // new assignment
print(m);