streaming using head and tail

[Matthias Brantner]

The optimizer should prevent materialization of a sequence if it's only consumed by head and tail as shown in the following example.

let $lines := 1 to 100000
let $header := head($lines)
for $line in tail($lines)
return { $header : $line }

Because head and tail are currently rewritten into subsequence it either requires the optimizer to detect the corresponding subsequence expressions, provide explicit iterator for head and tail, or use the code generation to produce code using subsequence.