Add some multiset-related lemmas #1329
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For a commutative fold_right operator, any folding order (i.e., folding over any permutation of a sequence) produces the same result.
lemma_fold_right_split (formerly aux_lemma_fold_right_alt) is a useful lemma in its own right about how fold_right distributes over splitting a sequence into two parts using subrange. lemma_fold_right_commute_one is also useful for proofs that need to commute the order of operations in fold_right, in a way that's not quite about permutations.
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The main lemmas of interest here are:
fold_right_permutationproves that, when doing afold_rightwith a commutative operator, any permutation of the sequence produces the same result.to_multiset_removestrengthensto_multiset_ensures()to reason about removing elements from a seq.lemma_multiset_has_no_duplicates_convis the converse oflemma_multiset_has_no_duplicatesand helps prove that, if a sequence wasno_duplicates(), then any permutation (with an equalto_multiset()) also hasno_duplicates().