x0 = [] ∨ x0 = y + a0 ' ⊦ ∃ y0, a1. a ' + x0 = y0 + a1 '
Used simplifier rules: c: ⊦ a + x = a ' + x;%e-01: ⊦ a ' + x ≠ [];%a: ⊦ (x + y) + z = x + y + z;