⊦ rmdup(a ' + x) = (a ∈ x ⊃ rmdup(x);a ' + rmdup(x))
a ∈ (a0 ∈ x0 ⊃ rmdup(x0);a0 ' + rmdup(x0)), ∀ a. a ∈ rmdup(x0) → a ∈ x0 ⊦ a = a0, a ∈ x0
Used terms: x0, a0