src/Pure/library.scala

 
     def subSequence(i: Int, j: Int): CharSequence =
       if (0 <= i && i <= j && j <= length) new Reverse(text, end - j, end - i)
       else throw new IndexOutOfBoundsException
 
-    override def toString: String = {
-      val buf = new StringBuilder(length)
-      for (i <- 0 until length)
-        buf.append(charAt(i))
-      buf.toString
-    }
+    override def toString: String =
+      string_builder(hint = length) { buf => for (i <- 0 until length) buf.append(charAt(i)) }
   }
 
   class Line_Termination(text: CharSequence) extends CharSequence {
     def length: Int = text.length + 1
     def charAt(i: Int): Char = if (i == text.length) '\n' else text.charAt(i)