[isabelle] Congruence rule for Let



Hi all,

using the function package, I'd like to define a function whose definition contains a number of Let expressions. In the generated induction rule, a term "Let t (%x. y)" yields the induction hypothesis "!!x. x = t ==> P (y x)" However, I would like to get "P (y t)" directly. How do I have to change the congruence rule for Let to achieve this?

I tried two alternatives with fundef_cong:
- "[| M = N; f N = g N |] ==> Let M f = Let N g" raises an exception:

  *** exception THM 1 raised (line 421 of "drule.ML"): COMP
  *** At command "function".

- "[| M = N; f M = g N |] ==> Let M f = Let N g" eliminates the quantifier, but produces far to many induction hypotheses.

What is the right congruence rule for this?

Andreas






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