さらに2019.2.16. 夜、計算機が ゼロ除算ができるとの情報が入り、本人と連絡が取れた: Added an answer In the proof assistant Isabelle/HOL we have x/0 = 0 for each number x. This is advantageous in order to simplify the proofs. You can download this proof assistant here: https://isabelle.in.tum.de/