Implement `choosePrime`

tp/rsa/main.hs

@@ -60,3 +60,6 @@ dividers n = [k | k <- takeWhile (\k -> k * k <= n) primeinf, rem n k == 0]
 prime n = null (dividers n)
 primeinf = 2:3:[n | n <- primecandidates, prime n]
 
+choosePrime::Int -> Int
+choosePrime b = head $ dropWhile (<= b) primeinf
+