Allow subclasses to customize backtracking behavior.
Allow subclasses to customize backtracking behavior.

tomfisher/tripleplay,tomfisher/tripleplay,tomfisher/tripleplay,joansmith/tripleplay,joansmith/tripleplay,joansmith/tripleplay,joansmith/tripleplay,tomfisher/tripleplay,joansmith/tripleplay,tomfisher/tripleplay