Used simplifier rules: Butmin: ⊦ st.butmin = st -- st.min;%min-def: ⊦ st ≠ ∅ → st.min = st.keys.min;% ⊦ ¬ a ∈ ∅; (s)% ⊦ a ∈ st -- b ↔ a ≠ b ∧ a ∈ st; (ws)