+
+
+
+/* If "err" != 0, exit with an error message "msg" and clean up. If "msg" is a
+ * NULL pointer, return generic error message. (For "err", pass the result of
+ * functions that return non-zero as an error status and thereby avoid bloating
+ * up the code with if-error-conditionals.)
+ */
+extern void exit_err(uint8_t err, struct World * world, char * msg);