code: def backsub_both_sides(L, X, transpose='left'):
      
      if transpose == 'left':
          tmp, _ = dtrtrs(L, X, lower=1, trans=1)
          return dtrtrs(L, tmp.T, lower=1, trans=1)[0].T
      else:
          tmp, _ = dtrtrs(L, X, lower=1, trans=0)
          return dtrtrs(L, tmp.T, lower=1, trans=0)[0].T
docs: Return L^-T * X * L^-1, assumuing X is symmetrical and L is lower cholesky