[isabelle-dev] Numerals in non-canonical form [was: NEWS]

[Florian Haftmann]

> This refers to Isabelle/e7c9a14632d0 and AFP/797f66d26443.
> 
> * Abolished neg_numeral.
>   * Canonical representation for minus one is "- 1".
>   * Canonical representation for other negative numbers is
>     "- (numeral _)".
>   * When devising rule sets for number calculation, consider the
>     following cases: 0, 1, numeral _, - 1, - numeral _.
>   * Syntax for negative numerals is mere input syntax.
> INCOMPATBILITY.

This also means that the set of numerals in non-canonical form has
increased: it does not contain only Numeral1 but also "- 0" and even
things like "- - - 2".

Currently HOLogic.dest_number recognizes all of these.  Maybe this very
liberal semantics deserves reconsideration.  Or maybe there should be
canonical normalizing conversion for numerals:

	Numeral1 = 1
	- 0 = 0
        - - 1 = 1
        - - numeral _ = numeral

Currently different packages have different opinions on that.

	Florian

-- 

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http://home.informatik.tu-muenchen.de/haftmann/pgp/florian_haftmann_at_informatik_tu_muenchen_de

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