add append function to tiny_rope.sml

proj.mlb

@@ -9,5 +9,5 @@ in
   automerge.sml
 end
 
-rope.sml
+tiny_rope.sml
 utils.sml

tiny_rope.sml

@@ -1,15 +1,16 @@
-signature ROPE =
+signature TINY_ROPE =
 sig
   type t
   val empty: t
   val fromString: string -> t
   val size: t -> int
   val insert: int * string * t -> t
+  val append: string * t -> t
   val delete: int * int * t -> t
   val toString: t -> string
 end
 
-structure Rope :> ROPE =
+structure TinyRope :> TINY_ROPE =
 struct
   datatype t =
     N0 of string
@@ -249,6 +250,49 @@ struct
        | DeletedNode => delRoot rope)
     end
 
+  fun app (newStr, rope) =
+    case rope of
+      N2 (l, lm, r) =>
+        let
+          val (r, action) = app (newStr, r)
+        in
+          (case action of
+             NoAction =>
+               (case (l, r) of
+                  (N0 s1, N0 s2) =>
+                    if isLessThanTarget (s1, s2) then
+                      (N0 (s1 ^ s2), DeletedNode)
+                    else
+                      (N2 (l, lm, r), action)
+                | _ => (N2 (l, lm, r), action))
+           | AddedNode => (insN2Right (l, r), action)
+           | DeletedNode => (delN2Right (l, r), action))
+        end
+    | N1 t =>
+        let
+          val (t, action) = app (newStr, t)
+        in
+          (case action of
+             AddedNode => (insN1 t, action)
+           | _ => (N1 t, action))
+        end
+    | N0 oldStr =>
+        if isLessThanTarget (oldStr, newStr) then
+          (N0 (oldStr ^ newStr), NoAction)
+        else
+          (L2 (oldStr, newStr), AddedNode)
+    | _ => raise AuxConstructor
+
+  fun append (str, rope) =
+    let
+      val (rope, action) = app (str, rope)
+    in
+      (case action of
+         NoAction => rope
+       | AddedNode => insRoot rope
+       | DeletedNode => delRoot rope)
+    end
+
   fun delLeaf (startIdx, endIdx, str) =
     if startIdx <= 0 andalso endIdx >= String.size str then
       (empty, false)

utils.sml

@@ -21,17 +21,17 @@ fun run_txns arr =
       let
         val rope = 
           if del_num > 0 
-          then Rope.delete(pos, del_num, rope)
+          then TinyRope.delete(pos, del_num, rope)
           else rope
         val str_size = String.size ins_str
         val rope = 
           if str_size > 0 
-          then Rope.insert(pos, ins_str, rope)
+          then TinyRope.insert(pos, ins_str, rope)
           else rope
       in
         rope
       end) 
-    Rope.empty arr
+    TinyRope.empty arr
 
 fun run_txns_time title arr = 
   let
@@ -41,7 +41,7 @@ fun run_txns_time title arr =
   end
 
 fun run_to_string rope =
-  Rope.toString rope
+  TinyRope.toString rope
 
 fun run_to_string_time title rope =
   let