Recursive DTD

[ReallyConfused]

This is our DTD:
<!ELEMENT container (class*, attribute*, attribute_list*, #PCDATA)>
<!ELEMENT class (name?, class*, attribute*,attribute_list*, #PCDATA)>
<!ELEMENT attribute (#PCDATA)>
<!ATTLIST attribute value CDATA #REQUIRED>
<!ATTLIST attribute attributeType CDATA #REQUIRED>

<!ELEMENT name (#PCDATA)>

<!ELEMENT attribute_list(class*, attribute*, #PCDATA) >
<!ATTLIST attribute_list attributeType CDATA #REQUIRED >

And when we try to validate some xml based on this dtd we get these errors:
node=class, error=at line number, 94: Element type &quot;attribute_list&quot; must be declared.
node=container, error=at line number, 28: Element type &quot;attribute_list&quot; must be declared.
at line number, 72: Element type &quot;attribute_list&quot; must be declared.
at line number, 81: Element type &quot;attribute_list&quot; must be declared.
node=attribute_list, error=at line number, 83: Element type &quot;class&quot; must be declared.

Is our DTD correct? If not, why not? If it is then why are we getting these errors??