  We revisit the problem of uniqueness for the Ricci flow and give a short,
direct proof, based on the consideration of a simple energy quantity, of
Hamilton/Chen-Zhu's theorem on the uniqueness of complete solutions of
uniformly bounded curvature. With a variation of this quantity and technique,
we further prove a uniqueness theorem for subsolutions to a general class of
mixed differential inequalities which implies an extension of Chen-Zhu's result
to solutions (and initial data) of potentially unbounded curvature.
