A guide to the reviewer: the only reference to khmer::read_parsers::IParser::ParserState::thread_id was in the  (uint32_t const thread_id, uint8_t const trace_level) ParserState constructor where the supplied thread_id is recorded in the TraceLogger in lib/read_parsers.cc:1446 https://github.com/ged-lab/khmer/blob/358675f38c84289c8f7bd579b1558237a42c092f/lib/read_parsers.cc#L1446