code: public void register(final String word, final String replacement) {
          ArgUtils.notEmpty(word, "word");
          ArgUtils.notNull(replacement, "replacement");
          
          if(word.length() == 1) {
              singles.computeIfAbsent(word.charAt(0), key -> replacement);
              
          } else {
              multi.add(new MultiChar(word, replacement));
          }
          
      }
docs: 置換対象の文字を登録する。
  @param word 置換対象の文字
  @param replacement 置換後の文字
  @throws IllegalArgumentException word is empty.
  @throws NullPointerException replacement is null.