code: public static String findStartsWithTerminalString(List<TerminalString> completionList) {
          StringBuilder builder = new StringBuilder();
          for (TerminalString completion : completionList)
              while (builder.length() < completion.getCharacters().length() &&
                      startsWithTerminalString(completion.getCharacters().substring(0, builder.length() + 1), completionList))
                  builder.append(completion.getCharacters().charAt(builder.length()));
  
          return builder.toString();
      }
docs: Return the biggest common startsWith string
  
  @param completionList list to compare
  @return biggest common startsWith string