diff --git a/src/Cryptol/PrimeEC.hs b/src/Cryptol/PrimeEC.hs
index 026f2f82d..e86771ba3 100644
--- a/src/Cryptol/PrimeEC.hs
+++ b/src/Cryptol/PrimeEC.hs
@@ -47,12 +47,6 @@ module Cryptol.PrimeEC
   ) where
 
 
-{-
-import           GHC.Num.BigNat (BigNat#)
-import qualified GHC.Num.Backend as BN
-import qualified GHC.Num.BigNat as BN
-import qualified GHC.Num.Integer as BN
--}
 import           GHC.Num.Compat (BigNat#)
 import qualified GHC.Num.Compat as BN
 import           GHC.Exts
diff --git a/src/GHC/Num/Compat.hs b/src/GHC/Num/Compat.hs
index 8c96a6a8a..84ed6abbe 100644
--- a/src/GHC/Num/Compat.hs
+++ b/src/GHC/Num/Compat.hs
@@ -52,6 +52,9 @@ import           GHC.Exts
 bigNatToInteger :: BigNat# -> Integer
 bigNatToInteger = Integer.integerFromBigNat#
 
+-- | @'integerRecipMod' x m@ computes the modular inverse of @x@ mod @m@.
+--
+-- PRECONDITION: @m@ must be strictly positive.
 integerRecipMod :: Integer -> Integer -> Maybe Integer
 integerRecipMod x y =
   case Integer.integerRecipMod# x (Integer.integerToNaturalClamp y) of
@@ -134,6 +137,9 @@ integerToBigNat (GMP.S# i)  = GMP.wordToBigNat (int2Word# i)
 integerToBigNat (GMP.Jp# b) = b
 integerToBigNat (GMP.Jn# b) = b
 
+-- | @'integerRecipMod' x m@ computes the modular inverse of @x@ mod @m@.
+--
+-- PRECONDITION: @m@ must be strictly positive.
 integerRecipMod :: Integer -> Integer -> Maybe Integer
 integerRecipMod x y
   | res == 0  = Nothing