18 lines
792 B
Text
18 lines
792 B
Text
-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
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-- Released under Apache 2.0 license as described in the file LICENSE.
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-- Author: Leonardo de Moura
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import classical hilbert decidable
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using decidable
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-- Excluded middle + Hilbert implies every proposition is decidable
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-- First, we show that (decidable a) is inhabited for any 'a' using the excluded middle
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theorem inhabited_decidable [instance] (a : Bool) : inhabited (decidable a)
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:= or_elim (em a)
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(assume Ha, inhabited_intro (inl Ha))
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(assume Hna, inhabited_intro (inr Hna))
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-- Note that inhabited_decidable is marked as an instance, and it is silently used
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-- for synthesizing the implicit argument in the following 'epsilon'
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theorem bool_decidable [instance] (a : Bool) : decidable a
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:= epsilon (λ d, true)
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