1. A classification system, comprising: a cascade of forward classification stages each configured to classify an instance by one of rejecting the instance, unconditionally accepting the instance, and conditionally accepting the instance, wherein the cascade is configured to generate for each instance rejected by at least one of the forward classification stages a respective classification record indicative of classification into a rejected class, generate for each instance unconditionally accepted by all of the forward classification stages a respective classification record indicative of classification into an accepted class, and conditionally accept each instance conditionally accepted by at least one of the forward classification stages; and a marginal instance classifier configured to generate for each of the instances conditionally accepted by the cascade a respective classification record indicative of classification into one of the rejected class and the accepted class.