docs: public void addAxisReflection(int axis) {
      assert (0 < axis && axis <= dim);
      // reset inverse transformation - needs recomputation.
      inv = null;
  
      // Formal:
      // Matrix homTrans = Matrix.unitMatrix(dim + 1);
      // homTrans[axis - 1][axis - 1] = -1;
      // trans = homTrans.times(trans);
      // Faster:
      for(int i = 0; i <= dim; i++) {
        trans[axis - 1][i] = -trans[axis - 1][i];
      }
    }