section ‹Subsequence Ordering›
theory Subseq_Order
imports Sublist
begin
text ‹
This theory defines subsequence ordering on lists. A list ‹ys› is a subsequence of a
list ‹xs›, iff one obtains ‹ys› by erasing some elements from ‹xs›.
›
subsection ‹Definitions and basic lemmas›
instantiation list :: (type) ord
begin
definition "xs ≤ ys ⟷ subseq xs ys" for xs ys :: "'a list"
definition "xs < ys ⟷ xs ≤ ys ∧ ¬ ys ≤ xs" for xs ys :: "'a list"
instance ..
end
instance list :: (type) order
proof
fix xs ys zs :: "'a list"
show "xs < ys ⟷ xs ≤ ys ∧ ¬ ys ≤ xs"
unfolding less_list_def ..
show "xs ≤ xs"
by (simp add: less_eq_list_def)
show "xs = ys" if "xs ≤ ys" and "ys ≤ xs"
using that unfolding less_eq_list_def by (rule subseq_order.antisym)
show "xs ≤ zs" if "xs ≤ ys" and "ys ≤ zs"
using that unfolding less_eq_list_def by (rule subseq_order.order_trans)
qed
lemmas less_eq_list_induct [consumes 1, case_names empty drop take] =
list_emb.induct [of "(=)", folded less_eq_list_def]
lemmas less_eq_list_drop = list_emb.list_emb_Cons [of "(=)", folded less_eq_list_def]
lemmas le_list_Cons2_iff [simp, code] = subseq_Cons2_iff [folded less_eq_list_def]
lemmas le_list_map = subseq_map [folded less_eq_list_def]
lemmas le_list_filter = subseq_filter [folded less_eq_list_def]
lemmas le_list_length = list_emb_length [of "(=)", folded less_eq_list_def]
lemma less_list_length: "xs < ys ⟹ length xs < length ys"
by (metis list_emb_length subseq_same_length le_neq_implies_less less_list_def less_eq_list_def)
lemma less_list_empty [simp]: "[] < xs ⟷ xs ≠ []"
by (metis less_eq_list_def list_emb_Nil order_less_le)
lemma less_list_below_empty [simp]: "xs < [] ⟷ False"
by (metis list_emb_Nil less_eq_list_def less_list_def)
lemma less_list_drop: "xs < ys ⟹ xs < x # ys"
by (unfold less_le less_eq_list_def) (auto)
lemma less_list_take_iff: "x # xs < x # ys ⟷ xs < ys"
by (metis subseq_Cons2_iff less_list_def less_eq_list_def)
lemma less_list_drop_many: "xs < ys ⟹ xs < zs @ ys"
by (metis subseq_append_le_same_iff subseq_drop_many order_less_le
self_append_conv2 less_eq_list_def)
lemma less_list_take_many_iff: "zs @ xs < zs @ ys ⟷ xs < ys"
by (metis less_list_def less_eq_list_def subseq_append')
lemma less_list_rev_take: "xs @ zs < ys @ zs ⟷ xs < ys"
by (unfold less_le less_eq_list_def) auto
end