public interface StringInverter {
        /** inverts a string
         */
        String invert(String str);
        /** splits a string in half and inverts
         * the two parts
         */
        default String invertHalf(String str) {
           int len = str.length();
           String str1 = str.substring(0, len/2);
           String str2 = str.substring(len/2, len);
           return str2+str1;
        }
        
        /** removes character at index from String str
         * and returns the new String.
         */
        static String removeChar(String str, int index) {
            if( str.length()-1 < index ) {
                return str;
            }
            String str1 = str.substring(0,index);
            String str2;
            if( str.length() >= index-1 ) {
                str2 = str.substring(index+1);
            }
            else {
                str2 = new String("");
            }
            return str1+str2;
        
        }
    }