It does NOT work at zone border in non symmorphic space groups.

As pointed out here, I noticed several edge cases which haven't been implemented in QE:

werner@X10DAi-00:~/q-e$ rg -i -A2 '\bzone border\b'
PHonon/PH/symmorphic_or_nzb.f90
12:  !! symmorphic or if the q point is not at zone border or if all the 
13-  !! phase factors \(e^{i G f}\) are equal to one. Presently the routine that
14-  !! finds the mode symmetry works only when this function is TRUE.

PP/src/sym_band.f90
180:           WRITE(stdout,'(/,5x,"zone border point and non-symmorphic group ")')
181-           WRITE(stdout,'(5x,"symmetry decomposition not available")')
182-           WRITE(stdout, '(/,1x,74("*"))')
--
318:  !   Presently it does NOT work at zone border if the space group of
319-  !   the crystal has fractionary translations (non-symmorphic space groups).
320-  !
--
680:  !   Presently it does NOT work at zone border if the space group of
681-  !   the crystal has fractionary translations (non-symmorphic space groups).
682-  !

PHonon/PH/find_mode_sym.f90
13:  !! matrix. It does NOT work at zone border in non symmorphic space groups.
14-  !! if flag=1 the true displacements are given in input, otherwise the
15-  !! eigenvalues of the dynamical matrix are given.  

PHonon/PH/obsolete.f90
1726:  !! matrix. It does NOT work at zone border in non symmorphic space groups.
1727-  !! if flag=1 the true displacements are given in input, otherwise the
1728-  !! eigenvalues of the dynamical matrix are given.

Why not deal with these situations so far? Are they very difficult to implement, or do they have relatively few applications?

Regards, HZ