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Proof step 4 from proof rev-tail (previous step)

rev(x) + y = a + y0 ⊦ rev(x) + y = [], x = [], y0 = rev(x).tail + y

Heuristic simplifier applied rule simplifier

And obtained the following new goals

Proof step 5:
rev(x) + y = a ' + y0, a ∈ x ∨ a ∈ y ⊦ x = [], y0 = rev(x).tail + y

Rule argument

Additional information

Used simplifier rules: rev-in: ⊦ a ∈ rev(x) ↔ a ∈ x;% ⊦ a ∈ x + y ↔ a ∈ x ∨ a ∈ y; (s)% ⊦ a ' + x ≠ []; (s)%a: ⊦ (x + y) + z = x + y + z; from specification list%in-14: ⊦ a ' + x = y → a ∈ y; (forward) from specification list% ⊦ a + x = a ' + x; (s)