The next letters on the list, if it comes to it, which it probably will, are epsilon, zeta, eta and theta. (B.1.1.7s new name is Alpha.) Currently, there are no plans on how to label the variants after the 24 letters of the Greek alphabet are used up. These old names are not abandoned. ALPHA, BETA, gamma, delta.
The next letters are all chicken.
( z0 chicken :domain ( z1 letter :mod ( z2 next ) :mod ( z3 all ) ) )
(z0 / chicken
    :domain (z1 / letter
                :mod (z2 / next)
                :mod (z3 / all)))
The next letters are all chicken .
z1 :mod z2
z1 :mod z3
z0 :domain z1