| ∃ a, b. a ≠ b ∧ s = ∅ ++ a ++ b ⊦ s.butmin.min ∈ s |
⊦ a = b, (∅ ++ a ++ b).butmin.min = b, (∅ ++ a ++ b).butmin.min = a
Used simplifier rules: in-04: ⊦ a ∈ ∅ ++ b ↔ a = b; from specification set-basic%add: ⊦ ¬ a ∈ s ++ b ↔ a ≠ b ∧ ¬ a ∈ s; from specification set-basic