ACTION: var evaluation

I had an action to update var evaluation.


It turns out that we already require declaration before use:


https://www.w3.org/community/xformsusers/wiki/XForms_2.0#Variable_Scope


Variable scoping is lexical: a variable defined by a var element is visible 
for all following sibling nodes and their descendants. The variable is not 
visible for the var element itself.


And we already disallow duplicate var names:


Variable shadowing is not allowed: if a variable is declared with a name 
that is already in scope at the point of the new variable declaration, an 
xforms-binding-error 
<https://www.w3.org/community/xformsusers/wiki/XForms_2.0#evt-bindingError> 
 is dispatched to the var element.


I have consequently updated the evaluation section to require 
document-order evaluation.


Steven