+extern void * try_malloc(size_t size, struct World * w, char * f);
+extern void * try_calloc(size_t nmemb, size_t size,
+ struct World * w, char * f);
+
+
+
+/* Check if tempfile "path" exists, and if so, exit with explanation that. */
+extern void check_tempfile(char * path, struct World * f);
+
+/* If one and only one of files at "p1", "p2" exists, fail with explanation. */
+extern void check_files_xor(char * p1, char * p2, struct World * w);