先ほどの方に メールしました。 Larry Paulson 氏の見解が、述べられていますが、 私の理解と同じで、彼らは、まだ真実を 知っていません。
下記の 文章です。
in 12 hours
José Manuel Rodríguez Caballero
Added an answer
I will reproduce here an email exchange with Prof. Lawrence Paulson (Cambridge) about the division by zero:
Extending the domain of division with x/0 = 0 (and also for integers) is a common choice in a great many systems, not just Isabelle/HOL but HOL Light, HOL4, MetiTarski and doubtless many other systems. They do it simply for convenience and not for any deep reason. The point is that many laws involving division will then hold unconditionally, an example being (a*b)/(c*d) = (a/c)*(b/d).
The technique of extending the domains of functions to simplify theorem proving goes back at least to the 1970s and the Boyer/Moore prover.