Computational complexity of accessing the Nth item in

Hi,

I wonder what should be the most likely computational complexity of:

     $sequence[$N]

and

    $node-set[$N]

Is it constant, such as 

      complexity(sequence[1]) = complexity(sequence[$N])   
                            for every $N in 1 to count(sequence)


or is 

    complexity(sequence[1000]) = 1000 * complexity(sequence[1])

Another question is whether the functions on sequences are faster that
manipulating them "by hand". I have some definite observations on
Saxon 8.2, but its behaviour may be atypical or may change in future
versions.


One translation in more practical terms: would it be realistic to try
to perform binary search in a sorted sequence?

Cheers,
Dimitre Novatchev.