⊦ a ∈ st.keys → (st -- a).keys = st.keys -- a
a ∈ st, a ∈ st.keys, # st.keys ≤ #(st.keys -- a) ⊦ a < st.min
Used simplifier rules: Keys: ⊦ a ∈ st.keys ↔ a ∈ st; Used terms: a, st