Short-circuit range counting algorithm?

[Andrei Alexandrescu]

On 3/16/18 1:41 PM, H. S. Teoh wrote:
> Given a forward range r, I want to test whether it has exactly n
> elements that satisfy some given predicate pred.  Is it possible to do
> this using current Phobos algorithms such that it does not traverse more
> members of the range than necessary?
> 
> The naïve solution `r.count!pred == n` walks the entire range, even
> though it may already have seen n+1 elements that satisfies pred, and
> therefore should already know that the answer must be false.

r.count!pred.walkLength(n) == n