  R. Crew conjectured that every overconvergent F-isocrystal over k((t)) (k a
field of positive characteristic) is quasi-unipotent (equivalently, potentially
semistable), and so has ``generic'' and ``special'' Newton polygons. It is easy
to construct a Newton polygon for an arbitrary overconvergent F-crystal that
coincides with the generic Newton polygon for potentially semistable crystals.
We give an analogous construction for the special Newton polygon, by showing
that the F-crystal can be trivialized over a large auxiliary ring. In a
subsequent preprint, we use this construction to prove the aforementioned
conjecture.
