code: public static String insert(final String value, final String substr, final int index) {
          validate(value, NULL_STRING_PREDICATE, NULL_STRING_MSG_SUPPLIER);
          validate(substr, NULL_STRING_PREDICATE, NULL_STRING_MSG_SUPPLIER);
          if (index > value.length()) {
              return value;
          }
          return append(value.substring(0, index), substr, value.substring(index));
      }
docs: Inserts 'substr' into the 'value' at the 'index' provided.
  
  @param value  The input String
  @param substr The String to insert
  @param index  The index to insert substr
  @return String with substr added