On October 5, 2016, the Bureau of Internal Revenue (BIR) sent KLM Corp. a Final Assessment Notice (FAN), stating that after its audit pursuant to a Letter of Authority duly issued therefore, KLM Corp. had deficiency value-added and withholding taxes. Subsequently, a warrant of distraint and/or levy was issued against KLM Corp. KLM Corp. opposed the actions of the BIR on the ground that it was not accorded due process because it did not even receive a Preliminary Assessment Notice (PAN) after the BIR's investigation, which the BIR admitted. Requirements: a) Distinguish a PAN from a FAN. b) Are the deficiency tax assessment and warrant of distraint and/or levy issued against KLM Corp. valid? Explain