XMI12 — General question to "3.6.4 Inheritance Specification"

General question to "3.6.4 Inheritance Specification": Why we declare all
XML elements and XML attributes direct under class C0 declaration and not as
parameter entity named like:

<!ENTITY % C0.elem "C0.a0">
<!ENTITY % C0.ref "C0.r0">
<!ENTITY % C0.comp "C0.comp0">

<!ENTITY % "C0.attr
a0 CDATA #IMPLIED
a1 CDATA #IMPLIED
r0 CDATA #IMPLIED
r1 CDATA #IMPLIED
>

and then:
<!ELEMENT C0 (%C0.elem; | XMI.extension | %C0.ref; | %C0.comp*>
<!ATTLIST C0
%C0.attr;
%XMI.element.att;
%XMI.link.att;"
>

and for third subclass analogical:

<!ELEMENT C3
(%C0.elem; | %C1.elem; | %C2.elem | %C3.elem | XMI.extension |
%C0.ref; | %C1.ref; | %C2.ref; | %C3.ref |
%C0.comp | %C1.comp | %C2.comp | %C3.comp )*>

• >
  <!ATTLIST C3
  %C0.attr;
  %C1.attr;
  %C2.attr;
  %C3.attr;
  %XMI.element.att;
  %XMI.link.att;"
  >

Imagine how short would be the DTD of UML in such an format.

It can be even shorter if we declare only one entity for all subelements
like:

<!ENTITY % C0.elem "C0.a0 | C0.r0 | C0.comp0 | XMI.extension">
<!ENTITY % C1.elem "%C0.elem | ... my own elements without XMI.extension ...
">

and than any n-subclass:

<!ENTITY Cn "%C'n-1'.elem | ... my own elements without XMI.extension ...">
<!ELEMENT Cn (%Cn.elem*>

analogical implified form for ATTLIST is applicable, the last example could
be:

<!ENTITY Cn.attr "
%C'n-1'.attr
... my own elements without %XMI.element.att; and %XMI.link.att; ... ">
<!ATTLIST Cn
%Cn.attr;
>