docs: public boolean isFullRank() {
      // Find maximum:
      double t = 0.;
      for(int j = 0; j < n; j++) {
        double v = Rdiag[j];
        if(v == 0) {
          return false;
        }
        v = Math.abs(v);
        t = v > t ? v : t;
      }
      t *= 1e-15; // Numerical precision threshold.
      for(int j = 1; j < n; j++) {
        if(Math.abs(Rdiag[j]) < t) {
          return false;
        }
      }
      return true;
    }