$ grep -i "element sg6" *.txt
091008-acc_200.dtd.txt:<!ELEMENT SG6 (RFF*, DTM*, MOA, FTX*)>
091009-acc_200.dtd.txt:<!ELEMENT SG6 (RFF*, DTM*, MOA, FTX*)>
091012-acc_200.dtd.txt:<!ELEMENT SG6 (PF:TGT*, RFF*, DTM*, MOA, PF:EPC*, FTX*)>
091013-acc_200.dtd.txt:<!ELEMENT SG6 (PF:TGT*, RFF*, DTM*, MOA, PF:EPC*, FTX*)>
091014-acc_200.dtd.txt:<!ELEMENT SG6 (PF:TGT*, RFF*, DTM*, MOA, PF:EPC*, FTX*)>
091031-acc_200.dtd.txt:<!ELEMENT SG6 (PF:TGT*, RFF*, DTM*, MOA, PF:EPC*, FTX*)>

$ diff 091008-acc_200.dtd.txt 091009-acc_200.dtd.txt

$ diff 091009-acc_200.dtd.txt 091012-acc_200.dtd.txt
35c35
< <!ELEMENT SG6 (RFF*, DTM*, MOA, FTX*)>
---
> <!ELEMENT SG6 (PF:TGT*, RFF*, DTM*, MOA, PF:EPC*, FTX*)>
36a37,41
> <!-- PF:EPC ******************************************* -->
> <!ELEMENT PF:EPC (PF:D_4752, PF:D_4753, PF:D_4754?)>
> <!ATTLIST PF:EPC
>       xmlns:PF CDATA #FIXED "http://www.post.ch/xml"
> >
41a47,51
> <!-- PF:TGT ******************************************* -->
> <!ELEMENT PF:TGT (PF:D_4752, PF:D_4753, PF:D_4754?)>
> <!ATTLIST PF:TGT
>       xmlns:PF CDATA #FIXED "http://www.post.ch/xml"
> >
90a101,116
> <!-- PF:D_4752 ******************************************* -->
> <!ELEMENT PF:D_4752 EMPTY>
> <!ATTLIST PF:D_4752
>       xmlns:PF CDATA #FIXED "http://www.post.ch/xml"
>       Value (EPC | TGT) #REQUIRED
> >
> <!-- PF:D_4753 ****************************************** -->
> <!ELEMENT PF:D_4753 (#PCDATA)>
> <!ATTLIST PF:D_4753
>       xmlns:PF CDATA #FIXED "http://www.post.ch/xml"
> >
> <!-- PF:D_4754 ****************************************** -->
> <!ELEMENT PF:D_4754 (#PCDATA)>
> <!ATTLIST PF:D_4754
>       xmlns:PF CDATA #FIXED "http://www.post.ch/xml"
> >

$ diff 091012-acc_200.dtd.txt 091013-acc_200.dtd.txt

$ diff 091013-acc_200.dtd.txt 091014-acc_200.dtd.txt

$ diff 091014-acc_200.dtd.txt 091031-acc_200.dtd.txt