docs: protected static String prefixCode(String code, int scale) {
          switch (scale) {
          case COIN_SCALE:      return code;
          case 1:               return "d" + code;
          case 2:               return "c" + code;
          case MILLICOIN_SCALE: return "m" + code;
          case MICROCOIN_SCALE: return "µ" + code;
          case -1:              return "da" + code;
          case -2:              return "h" + code;
          case -3:              return "k" + code;
          case -6:              return "M" + code;
          default: throw new IllegalStateException("No known prefix for scale " + String.valueOf(scale));
          }
      }