src/HOL/ZF/HOLZF.thy

 lemma Fun_total: "\<lbrakk>Elem F (Fun U V); Elem a U\<rbrakk> \<Longrightarrow> \<exists>x. Elem (Opair a x) F"
   using [[simp_depth_limit = 2]]
   by (auto simp add: Fun_def Sep Domain)
 
 
-lemma unique_fun_value: "\<lbrakk>isFun f; Elem x (Domain f)\<rbrakk> \<Longrightarrow> ?! y. Elem (Opair x y) f"
+lemma unique_fun_value: "\<lbrakk>isFun f; Elem x (Domain f)\<rbrakk> \<Longrightarrow> \<exists>!y. Elem (Opair x y) f"
   by (auto simp add: Domain isFun_def)
 
 lemma fun_value_in_range: "\<lbrakk>isFun f; Elem x (Domain f)\<rbrakk> \<Longrightarrow> Elem (f\<acute>x) (Range f)"
   apply (auto simp add: Range)
   apply (drule unique_fun_value)