vector multiply(matrix& m, vector& v);
  {
      vector r;
      for (int i = 0; i<3; i++) { // r[i]="m[i]" * v; r.elem(i)="0;" for (int j="0;" j<3; j++) r.elem(i) +="m.elem(i,j)" * v.elem(j); } return r; }