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Note that during the transition period tstring is typedef'ed to
std::string.

See: https://github.com/tensorflow/community/pull/91

example_pos_ is the cursor to current position when reading the corpus file. When corpus file is very big, the corpus_size_ may bigger than max of int32, then example_pos_ became negative, and cause `corpus_[example_pos_]` to `Segmentation Fault`.