code: function swapRowsInPlace (M, i, j) {
    const me = M.elements;
    let a1 = me[i];
    let a2 = me[i + 3];
    let a3 = me[i + 6];
  
    me[i] = me[j];
    me[i + 3] = me[j + 3];
    me[i + 6] = me[j + 6];
  
    me[j] = a1;
    me[j + 3] = a2;
    me[j + 6] = a3;
  }
docs: Swaps two rows in the matrix.
  @memberof Helpers
  @param {Matrix2|Matrix3|Matrix4} M The matrix.
  @param {number} i The first row index.
  @param {number} j The second row index.