diff --git a/doc/glossary.sgml b/doc/glossary.sgml
index 23fd8ba..f14d657 100644
--- a/doc/glossary.sgml
+++ b/doc/glossary.sgml
@@ -1,4 +1,4 @@
-<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook V3.1//EN">
+<!-- <!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook V3.1//EN"> -->
 
 <glossary>
   <title>
diff --git a/doc/install.sgml b/doc/install.sgml
index 5f54875..dfe47ba 100644
--- a/doc/install.sgml
+++ b/doc/install.sgml
@@ -1,5 +1,4 @@
-<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook V3.1//EN">
-
+<!-- <!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook V3.1//EN"> -->
 
 <section>
 <title>
diff --git a/doc/intro.sgml b/doc/intro.sgml
index 125bdb5..a564efc 100644
--- a/doc/intro.sgml
+++ b/doc/intro.sgml
@@ -1,5 +1,4 @@
-<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook V3.1//EN">
-<!--ignore the error caused by previous line-->
+<!-- <!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook V3.1//EN"> -->
 
 <section>
 
diff --git a/doc/other.sgml b/doc/other.sgml
index e087502..1b5260d 100644
--- a/doc/other.sgml
+++ b/doc/other.sgml
@@ -1,5 +1,4 @@
-<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook V3.1//EN">
-
+<!-- <!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook V3.1//EN"> -->
 
 
 <section>
diff --git a/doc/reference.sgml b/doc/reference.sgml
index 6abd827..1075008 100644
--- a/doc/reference.sgml
+++ b/doc/reference.sgml
@@ -1,4 +1,4 @@
-<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook V3.1//EN">
+<!-- <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook V3.1//EN" "docbook/dtd/3.1/docbook.dtd">  -->
 
 <section>
 <title>
diff --git a/doc/tutorial.sgml b/doc/tutorial.sgml
index 9b829d7..e805d49 100644
--- a/doc/tutorial.sgml
+++ b/doc/tutorial.sgml
@@ -1,5 +1,4 @@
-<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook V3.1//EN">
-
+<!-- <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook V3.1//EN" "docbook/dtd/3.1/docbook.dtd">  -->
 
 <section>
 <title>