rP(r)(mk(rs, f)), ∀ r. r ∈ rs → g(r) = a ⊦ ∃ r. rP(r)(mk(rs, f))
rP(r)(mk(rs, f)), r0 ∈ rs, ∀ r. r ∈ rs → g(r) = a ⊦ g(r0) = a