| a ∈ y, ∀ y. filter(rmdup(x0), y) = rmdup(filter(x0, y)) ⊦ filter(rmdup(a ' + x0), y) = a ' + rmdup(filter(x0, y)), a ∈ filter(x0, y) |
⊦ a ∈ filter(x, y) ↔ a ∈ x ∧ a ∈ y
a ∈ y, ∀ y. filter(rmdup(x0), y) = rmdup(filter(x0, y)) ⊦ filter(rmdup(a ' + x0), y) = a ' + rmdup(filter(x0, y)), a ∈ x0 ∧ a ∈ y
Used terms: y, x0, a