Another type of clone arose from developers in the open source and indie game venues, where the developers seek to recreate the gameplay of a popular title through reverse engineering, adding their own creative assets, and releasing the game typically for free and in homage to the original title. This allows the teams and users to expand upon original elements of the commercial game, such as software bugs that were not fixed, improving gameplay concepts, support for newer computers or console systems, or adding new ideas to the base gameplay principles, as well as enabling gameplay extensions through user-generated mods or add-ons. Some examples of these clones include "Freeciv" based on the "Civilization" series, "Osu!" based on "Osu! Tatakae! Ouendan", and "Frets on Fire" based on "Guitar Hero". The open source nature of these clones also enable new utilities, such as developing artificial intelligence agents that have learned and improved their play in "Freeciv" which in turn can help advance artificial intelligence research. Such games must be careful not to use the original game's assets or could face legal issues. "OpenSC2K", an open-source recreation of "SimCity 2000", was shut down by Electronic Arts after it was found that "OpenSC2K" used assets from "SimCity 2000". In the area of indie games, while there may be cloning of such games, indie developers tend to rely on an informal code of honor to shun those that do engage in cloning.