code: public JsonObject append(String name, Object value) throws JsonException {
          checkIfFrozen();
          Object current = nameValuePairs.get(checkName(name));
  
          final JsonArray array;
          if (current instanceof JsonArray) {
              array = (JsonArray) current;
          } else if (current == null) {
              JsonArray newArray = new JsonArray();
              nameValuePairs.put(name, newArray);
              array = newArray;
          } else {
              throw new JsonException("Key " + name + " is not a JsonArray");
          }
  
          array.put(value);
  
          return this;
      }
docs: Appends values to the array mapped to {@code name}. A new {@link JsonArray}
  mapping for {@code name} will be inserted if no mapping exists. If the existing
  mapping for {@code name} is not a {@link JsonArray}, a {@link JsonException}
  will be thrown.
  
  @throws JsonException if {@code name} is {@code null} or if the mapping for
  {@code name} is non-null and is not a {@link JsonArray}.
  @hide