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Proof step 14 from proof del-04 (previous step)

a ∈ x, perm(y, x), ∀ a. a ∈ x ∨ a ∈ y → #oc(a, x) = #oc(a, y), a ∈ x -1l a ∨ a ∈ y -1l a ⊦ #oc(a, x) -1 = #oc(a, y -1l a)

Heuristic simplifier applied rule simplifier

And obtained the following new goals

Proof step 15:
a ∈ x, perm(y, x), ∀ a. a ∈ x ∨ a ∈ y → #oc(a, x) = #oc(a, y), a ∈ x -1l a ∨ a ∈ y -1l a ⊦ #oc(a, x) - 1 = #oc(a, y -1l a)

Rule argument

Additional information

Used simplifier rules: pred-03: ⊦ n -1 = n - 1; from specification nat