Error while creating DTD file

Question

I want to create a DTD file but I unfortunately I am getting an error, please advice. Here is my code

<!ELEMENT SUB_CONDITION EMPTY >
<!ATTLIST SUB_CONDITION relation (=|<>|>|<|>=|<=|=Mask|<>Mask) #REQUIRED>

the error is saying: "The name token is required in the enumerated type list for the "relation" attribute declaration."

Answer 1

The <, =, and > characters (#x003C, #x003D, #x003E) are not allowed in attribute values declared as enumerated.

In an attribute declared to have enumerated values, the values must match Nmtoken (one or more NameChars). That excludes several characters, including <, =, and >, which are not part of the definition of NameChar.

< is especially troublesome (it is the start-of-tag delimiter in XML markup) and is not allowed in any attributes.