Health and Safety Code section 11383, subdivision (f), which provides that possession of precursors to manufacture hydriotic acid “shall be deemed” to be possession of the derivative substance, does not create an improper mandatory presumption. In fact, the statute creates no presumption at all, but is simply a valid exercise of the Legislature’s power to create substantive law and define crimes. The statute does not define an ultimate fact to be presumed from one or more basic facts, but instead deems the possession of the precursors to be the legal equivalent of possession of the derivative substance.