+/* Return 1 if cell at "test_pos" is proliferable by "t", i.e. it is passable,
+ * it is not inhabited by another thing of "t"'s type, and, if "t" is animate,
+ * neither by any other animate thing; else return 0.
+ */
+static uint8_t cell_is_proliferable(struct yx_uint8 test_pos, struct Thing * t);