-void run_shell ( const char *shell, int loginshell, const char *command, const char **additional_args )
+/* Run SHELL, or DEFAULT_SHELL if SHELL is "" or NULL.
+ * If COMMAND is nonzero, pass it to the shell with the -c option.
+ * If ADDITIONAL_ARGS is nonzero, pass it to the shell as more
+ * arguments. */
+void FAST_FUNC run_shell(const char *shell, int loginshell, const char *command, const char **additional_args)