  The trapezoidal Fox conjecture states that the coefficient sequence of the
Alexander polynomial of an alternating knot is unimodal. We are motivated by a
harder question, the strong Fox conjecture, which asks whether the coefficient
sequence of the Alexander polynomial of alternating knots is actually
log-concave. Our approach is to introduce a polynomial $\Delta(t)$ associated
to a Christoffel word and to prove that its coefficient sequence is
log-concave. This implies the strong Fox conjecture for two-bridge knots.
