src/Pure/General/table.ML

   exception DUP of key
   exception SAME
   exception UNDEF of key
   val empty: 'a table
   val is_empty: 'a table -> bool
+  val is_single: 'a table -> bool
   val map: (key -> 'a -> 'b) -> 'a table -> 'b table
   val fold: (key * 'b -> 'a -> 'a) -> 'b table -> 'a -> 'a
   val fold_rev: (key * 'b -> 'a -> 'a) -> 'b table -> 'a -> 'a
   val dest: 'a table -> (key * 'a) list
   val keys: 'a table -> key list

   Branch3 of 'a table * (key * 'a) * 'a table * (key * 'a) * 'a table;
 
 exception DUP of key;
 
 
-(* empty *)
+(* empty and single *)
 
 val empty = Empty;
 
 fun is_empty Empty = true
   | is_empty _ = false;
+
+fun is_single (Branch2 (Empty, _, Empty)) = true
+  | is_single _ = false;
 
 
 (* map and fold combinators *)
 
 fun map_table f =