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