public class MutableString
{
        private String contents = "";
        
        public MutableString append(String s) 
        {
                System.out.println("append, parameter: " + s);
                System.out.println("append, old contents: " + contents);
                contents = contents + s;
                System.out.println("append, new contents: " + contents);
                return this;
        }
        
        public char charAt(int index) 
        {
                return '\0';
        }
        
        public int indexOf(String s) 
        {
                return -1;
        }
        
        public MutableString insert(String s) 
        {
                return null;
        }
        
        public int length() 
        {
                return contents.length();
        }
        
        public MutableString replace(int start, int end, String s) 
        {
                return null;
        }
        
        public String substring(int start, int end) 
        {
                return null;
        }
        
        public String toString() 
        {
                return contents;
        }
}