in RELAX NG:
include "tei.rnc"
include "core.rnc"
include "header.rnc"
include "textstructure.rnc"
soCalled = notAllowed
start = TEI
in DTD:
<!DOCTYPE TEI
PUBLIC "-//TEI P5//DTD Main Document Type//EN" "tei.dtd" [
<!ENTITY % TEI.header 'INCLUDE' >
<!ENTITY % TEI.core 'INCLUDE' >
<!ENTITY % TEI.textstructure 'INCLUDE'
<!ENTITY % soCalled "IGNORE" >
]>
