1 INTRODUCTION  
2 NOTIONS AND CATEGORIZATION  
3 FEATURE ATTRIBUTION  
4 FREE-TEXT EXPLANATIONS  
5 ADVERSARIAL EXAMPLES  
6 AXIOMATIC ANALYSIS OF TEXT RANKING MODELS  
7 PROBING AND PARAMETRIC ANALYSIS OF TEXT RANKING MODELS  
8 EXPLAINABLE-BY-ARCHITECTURE MODELS  
9 RATIONALE-BASED METHODS  
10 LIMITATIONS AND OPEN QUESTIONS  
11 CONCLUSION  