docs: public static DMatrixSparseCSC triangle( boolean upper , int N , double minFill , double maxFill , Random rand ) {
          int nz = (int)(((N-1)*(N-1)/2)*(rand.nextDouble()*(maxFill-minFill)+minFill))+N;
  
          if( upper ) {
              return triangleUpper(N,0,nz,-1,1,rand);
          } else {
              return triangleLower(N,0,nz,-1,1,rand);
          }
      }