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Proof step 2 from proof perm-del-in (previous step)

perm(x -l a, y), a ∈ y, perm(y, x -l a) ⊦

User applied rule insert rewrite lemma

And obtained the following new goals

Proof step 3:
⊦ a ∈ y ∧ ¬ a ∈ x → ¬ perm(x, y)

Rule argument

spec: inst: name: in seq: ⊦ a ∈ y ∧ ¬ a ∈ x → ¬ perm(x, y) subst: [a, y, x] → [a, y, x -l a] flags: rewrite, current

Additional information

Used simplifier rules: in-02: ⊦ ¬ a ∈ x -l a; from specification list-del Used terms: a, y, x -l a