rmdup(x) = y + a ' + z, a ∈ rmdup(x) ⊦ a ∈ x
rmdup(x) = y + a ' + z, a ∈ y, a ∈ rmdup(x) ⊦
rmdup(x) = y + a ' + z, a ∈ z, a ∈ rmdup(x) ⊦