Altu wrote:
> Hi Peter,
> 
> Thanks for the reply.
> 
> The thing that was confusing me was that why would we insert a
> predicate that starts with / or // in a specifiec location in the
> XPath query. Looks like _generally_ they can be anywhere in the
> query and yet we will get the same result.

A predicate which immediately references the root element can be 
considered global in scope, in the sense that if it always evaluates to 
false (as is the case with your example referencing a root element type 
that could not exist), then the entire XPath statement which depends on 
it will necessarily also always be false. In that case it is immaterial 
where you place it in the expression.

Don't confuse syntax with logic. The XPath syntax permits you to express 
constraints in a variety of ways in order to direct the expression to 
reference the node that you want. Just because it allows something 
doesn't mean that it will always be meaningful to express it that way.

The same applies in many languages: I can write "if(1==0)" or its 
equivalent in most programming languages and it will be accepted for 
compilation even though it will always evaluate to false.

The only occasion on which I tend to write a predicate which immediately 
references the root element type is when I use it as a crude switch to 
detect the current document type in XSLT which is common to several. So 
for example in XSLT which handles DocBook, [/article] might enable a 
certain XPath which would otherwise not be needed in books, etc. It's 
inelegant and probably inefficient, and there are probably better 
examples from other fields.

///Peter
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