code: public static String findStartsWithOperation(List<? extends CompleteOperation> coList) {
          List<String> tmpList = new ArrayList<>();
          for (CompleteOperation co : coList) {
              String s = findStartsWith(co.getFormattedCompletionCandidates());
              if (s.length() > 0)
                  tmpList.add(s);
              else
                  return "";
          }
          return findStartsWith(tmpList);
      }
docs: If there is any common start string in the completion list, return it
  
  @param coList completion list
  @return common start string