L3 Core: Attr.isId(), Element.setIdAttribute(), Element.setIdAttributeNS(), setIdAttributeNode()

isId() is a declared as a method, though it would be more appropriate as 
an attribute.  If made a read-write attribute, then 
Element.setIdAttribute(), Element.setIdAttributeNS() and 
Element.setAttributeNode() could be eliminated since you could do the 
equivalent using Element.getAttributeNode[NS]().isId = true | false.