docs: @NonNull
      public static String appendPath(@NonNull String first,
                                      @NonNull String second) {
          String result = first + SEP + second;
  
          while (result.contains("//")) {
              result = result.replaceAll("//", "/");
          }
  
          if (result.length() > 1 && result.endsWith(SEP)) {
              return result.substring(0, result.length() - 1);
          } else {
              return result;
          }
      }