RE: when to use 'as' attribute on a variable

> xslt/xpath does use "tree" informally to include document 
> fragments that do
> not start with / 

No, this is not an "informal" usage, it's the correct usage. XPath Data
Model section 2.1

http://www.w3.org/TR/xpath-datamodel/

Every node belongs to exactly one tree, and every tree has exactly one root
node.

[Definition: A tree whose root node is a Document Node is referred to as a
document.]

Michael Kay
http://www.saxonica.com/