Proper dihedrals in Fourier function

GROMACS version: 2020.1
GROMACS modification: No
In GROMACS manual (in bonded interaction section), equation of Proper Dihedrals (using Fourier function) contains 4 coefficients (C1, C2, C3, C4), whereas, in GROMACS topology file formats section, for this type of proper dihedrals (Fourier Dihedral), order of parameters are given as C1, C2, C3, C4, C5. I would like to know, from where “C5” came ?
Your suggestion is welcome.

As noted in the manual, C5 = 0.

Comments are sometimes a bit odd and may not actually mean anything in practice, though obviously we try to avoid anything erroneous. But specifying the C5 term is not necessary (though the comment implies perhaps a zero can be explicitly given, but obviously isn’t necessary).

Dear Dr. Justin A. Lemkul,

Thanks you very much for your answer. It clarified my doubt.