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Proof step 2 from proof len-last-rest (previous step)

x + a ' = a0 + y ⊦ # y = # x, x + a ' = []

Heuristic simplifier applied rule simplifier

And obtained the following new goals

Proof step 3:
x + a ' = a0 ' + y, a = a0 ∨ a ∈ y, a0 ∈ x ∨ a0 = a ⊦ # y = # x

Rule argument

Additional information

Used simplifier rules: c: ⊦ a + x = a ' + x;%e-02: ⊦ x + a ' ≠ [];%a: ⊦ (x + y) + z = x + y + z;%in-15: ⊦ x + a ' = y → a ∈ y;%in-14: ⊦ a ' + x = y → a ∈ y;%in-append: ⊦ a ∈ x + y ↔ a ∈ x ∨ a ∈ y;%in-17: ⊦ a ∈ b ' ↔ a = b;