88     I &id_ref = entry2id_[entry];  variable
89     if (id_ref == 0) {  // T not found
92         id_ref = id2entry_.size();
97     return id_ref - 1;  // NB: id_ref = ID + 1
297     I &id_ref = fp2id_[fp];  variable
298     if (id_ref == 0) {  // T not found
301         id_ref = id2entry_.size();
306     return id_ref - 1;  // NB: id_ref = ID + 1
492     I &id_ref = entry2id_[entry];  variable
493     if (id_ref == 0) {  // T not found
496         id_ref = id2entry_.size() + first_;
501     return id_ref - 1;  // NB: id_ref = ID + 1