<?xml version="1.0"?>

<!DOCTYPE any [

<!ELEMENT el EMPTY>
<!ATTLIST el        
          nmtoken   NMTOKEN   #FIXED "a"
          nmtokens  NMTOKENS  #FIXED "a b c"
>
<!ELEMENT any ANY>
]>

<any>
  <el nmtoken="
a
"/>
  <el nmtokens="
a
b
c
"/>
</any>