This class defines an attribute for an element in a XSLT stylesheet,
is meant to reflect the structure defined in http://www.w3.org/TR/xslt#dtd, and the
mapping between Xalan classes and the markup attributes in the element.
Return a string that should represent the setter method.
The setter method name will be created algorithmically the
first time this method is accessed, and then cached for return
by subsequent invocations of this method.
String that should represent the setter method which which
may be used on objects to set a value that represents this attribute,
of null if no setter method should be called.