To modify the dtd
Define your modifications in a pair
of extension files
<!DOCTYPE TEI.2 SYSTEM "tei2.dtd" [
  <!ENTITY % TEI.prose    "INCLUDE" >
  <!ENTITY % TEI.extensions.ent
                SYSTEM "myMods.ent" >
  <!ENTITY % TEI.extensions.dtd
                SYSTEM "myMods.dtd" >
]><TEI.2>...</TEI.2>
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