Commit 2022-02-21 11:38 f66a5ddb

chore(data/set/basic): add a few lemmas and a @[simp] attribute (#12176)

• rename set.exists_eq_singleton_iff_nonempty_unique_mem to set.exists_eq_singleton_iff_nonempty_subsingleton, use set.subsingleton in the statement;
• add @[simp] to set.subset_compl_singleton_iff;
• add set.diff_diff_right_self.