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    <p><strong>Short Story:</strong> Use <code>find_consts</code></p> <p><strong>Long Story:</strong></p> <p><em>This a How-To to conquer such problems.</em></p> <p>In <code>Main</code>, there is <code>List.dropWhile</code></p> <pre><code>List.dropWhile :: "('a =&gt; bool) =&gt; 'a list =&gt; 'a list" </code></pre> <p>However, it only removes from the beginning. This may not be the intended function.</p> <pre><code>value "List.dropWhile (λ x. x = ''c'') [''c'', ''c'', ''d'']" "[''d'']" value "List.dropWhile (λ x. x = ''c'') [''d'', ''c'', ''c'']" "[''d'', ''c'', ''c'']" </code></pre> <p><strong>Manual Approach</strong></p> <p>We can write a function ourselves which removes all occurrences</p> <pre><code>fun dropAll :: "('a =&gt; bool) =&gt; 'a list =&gt; 'a list" where "dropAll P [] = []" | "dropAll P (x # xs) = (if P x then dropAll P xs else x # (dropAll P xs))" </code></pre> <p><strong>Searching the Library</strong></p> <p>However, this function is equivalent to filtering with <code>¬ P</code></p> <p>How can we find such library functions?</p> <p>If we know the signature of what we want to do, we can use <code>find_consts</code></p> <pre><code>find_consts "('a ⇒ bool) ⇒ 'a list ⇒ 'a list" </code></pre> <p>It returns 3 functions from Main, with that signature: <code>List.dropWhile</code>, <code>List.filter</code>, <code>List.takeWhile</code></p> <p>Now, let's show that we don't need <code>dropAll</code> but can do the same with <code>filter</code>.</p> <pre><code>lemma "dropAll P l = filter (λ x. ¬ P x) l" apply(induction l) by simp_all </code></pre> <p>It is advisable not to implement things like <code>dropAll</code>yourself but rather use filter. Thus, all lemmata proven for <code>filter</code> are usable.</p> <p><strong>Hints</strong></p> <p>Hint: we can use the convenient list comprehension syntax to write e.g. filter expressions</p> <pre><code>lemma "filter (λ x. ¬ P x) l = [x ← l. ¬ P x]" by simp </code></pre>
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    1. POHow can I remove all elements in a list with a certain property?
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    1. CO`find_consts` should be the first step when searching for existing functions (most of the time you have a rather specific idea of the signature and/or possible names). This is a bit lost in your long exposition. How about moving the hint to `find_consts` to the very top?
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