  We solve the rotational abrasion model of Roth, Marques and Durian
(arXiv:1009.3492), a one-dimensional quasilinear partial differential equation
resembling the inviscid Burgers equation with the unusual feature of a step
function factor as a coefficient. The complexity of the solution is primarily
in keeping track of the cases in the piecewise function that results from
certain amputation and interpolation processes, so we also extract from it a
model of an evolving planar tree graph that tracks the evolution of the coarse
features of the contour.
