# Theory Collecting1

subsection "A small step semantics on annotated commands"
theory Collecting1
imports Collecting
begin
text‹The idea: the state is propagated through the annotated command as an
annotation \<^term>‹{s}›, all other annotations are \<^term>‹{}›. It is easy
to show that this semantics approximates the collecting semantics.›
lemma step_preserves_le:
"⟦ step S cs = cs; S' ⊆ S; cs' ≤ cs ⟧ ⟹
step S' cs' ≤ cs"
by (metis mono2_step)
lemma steps_empty_preserves_le: assumes "step S cs = cs"
shows "cs' ≤ cs ⟹ (step {} ^^ n) cs' ≤ cs"
proof(induction n arbitrary: cs')
case 0 thus ?case by simp
next
case (Suc n) thus ?case
using Suc.IH[OF step_preserves_le[OF assms empty_subsetI Suc.prems]]
by(simp add:funpow_swap1)
qed
definition steps :: "state ⇒ com ⇒ nat ⇒ state set acom" where
"steps s c n = ((step {})^^n) (step {s} (annotate (λp. {}) c))"
lemma steps_approx_fix_step: assumes "step S cs = cs" and "s ∈ S"
shows "steps s (strip cs) n ≤ cs"
proof-
let ?bot = "annotate (λp. {}) (strip cs)"
have "?bot ≤ cs" by(induction cs) auto
from step_preserves_le[OF assms(1)_ this, of "{s}"] ‹s ∈ S›
have 1: "step {s} ?bot ≤ cs" by simp
from steps_empty_preserves_le[OF assms(1) 1]
show ?thesis by(simp add: steps_def)
qed
theorem steps_approx_CS: "steps s c n ≤ CS c"
by (metis CS_unfold UNIV_I steps_approx_fix_step strip_CS)
end