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Proof step 9 from proof perm-last-rec (previous step)

perm(x -1l a0, y0 + a ') ↔ a ∈ x -1l a0 ∧ perm((x -1l a0) -1l a, y0) ⊦ a0 ∈ x ∧ perm(x -1l a0, y0 + a ') ↔ a ∈ x ∧ a0 ∈ x -1l a ∧ perm((x -1l a0) -1l a, y0)

User applied rule cut formula

And closed the goal!

Rule argument

fma: a = a0

Additional information

Used simplifier rules: del1-in: ⊦ a ≠ b → (a ∈ x -1l b ↔ a ∈ x); from specification list-del Used formulas: a = a0