Implement `prime`

tp/rsa/main.hs

@@ -57,5 +57,6 @@ splitBlocks msg = concat (map ungroupBytes msg)
 -- Reuse arithmetics from slide 42
 primecandidates = [6 * k + a | k <- [1..], a <- [-1, 1]]
 dividers n = [k | k <- takeWhile (\k -> k * k <= n) primeinf, rem n k == 0]
-primeinf = 2:3:[n | n <- primecandidates, null (dividers n)]
+prime n = null (dividers n)
+primeinf = 2:3:[n | n <- primecandidates, prime n]