+/* Return yx_uint16 coordinate one step from "yx" in direction "dir" (east: 'E',
+ * west: 'W', north: 'N', south: 'S'). If "dir" is invalid or would wrap the
+ * move around the edge of a 2^16x2^16 cells field, "yx" remains unchanged.
+ */
+extern struct yx_uint16 mv_yx_in_dir(char dir, struct yx_uint16 yx);