Robinson defines five distinct defense categories, which he names failure of proof defenses, offense modification defenses, justifications, excuses, and nonexculpatory public policy defenses. Robinson and others typically describe these types as having a logical relationship to each other; Robinson characterizes this relationship as serial. He also states that each successive type of defense need only be considered if those preceding it are unavailable. However, this approach has several disadvantages and, arguably, no advantages. Instead of a serial approach, defendants' pleas should not be interpreted as conceding either the existence or the nonexistence of another possible defense. Footnotes