It is recommended programming practice to give the Prolog source programs the suffix .pl, or .ecl if it contains ECLiPSe specific code. It is not enforced by the system, but it simplifies managing the source programs. The compile/1 predicate automatically adds the suffix to the filename, so that it does not need to be specified; if the literal filename can not be found, the system tries appending each of the valid suffixes in turn and tries to find the resulting filename.