You should pass all the information the function needs (unless it's
available as a global variable) in the arguments to the function call.

We banned passing the context implicitly because it prevents many
optimizations. For example, if you're doing lazy evaluation, then you
typically have to save all aspects of the context that an expression
depends on. Saving the context "just in case" the function uses it is
very expensive. It also means you can't move function calls out of loops
and predicates such as //a[f() = 2].