The tenth problem in Project Euler revisits the prime numbers:

Calculate the sum of all the primes below two million.

Using the Sieve of Eratosthenes sequence developed earlier we can write the solution like this:

sieve 2L Map.empty |> 
    Seq.takeWhile (fun n -> n < 2000000L) |>
    Seq.sum |>
    printfn "sum = %A"

Arguably there is some inefficiency in generating a subsequence just consisting of the primes less than two million and then summing that sequence. I think the clarity of the resulting code makes up for the slight inefficiency.