Analisis perbandingan kebijakan PPN atas impor dan penyerahan alat angkutan tertentu dan penyerahan jasa kena pajak terkait alat angkutan pada perusahaan pelayaran niaga nasional: studi kasus: PT X = Comparative analysis on the import vat policy and supplies of certain transport equipment and supplies of services related to the transportation equipment taxable on national commerce shipping company; case study PT X