code: public void appendTo(Appendable app) throws IOException {
          app.append("{properties=[");
          int length = mProperties.length;
          for (int i=0; i<length; i++) {
              if (i > 0) {
                  app.append(", ");
              }
              app.append(mDirections[i].toCharacter());
              app.append(mProperties[i].getName());
          }
          app.append(']');
          app.append(", unique=");
          app.append(String.valueOf(isUnique()));
          app.append('}');
      }
docs: Appends the same results as toString, but without the "StorableIndex"
  prefix.