diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,27 +1,34 @@
-# Changelog for `hspray`
-
-All notable changes to this project will be documented in this file.
-
-The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
-and this project adheres to the
-[Haskell Package Versioning Policy](https://pvp.haskell.org/).
-
-
-## 0.1.0.0 - 2022-12-11
-
-First release.
-
-
-## 0.1.1.0 - 2022-12-12
-
-* New functions `toList`, `sprayTerms` and `bombieriSpray`.
-
-* New operation `.^`, to multiply a spray by an integer.
-
-* Added some unit tests.
-
-
-## 0.1.2.0 - 2023-02-24
-
-New function `derivSpray`, to differentiate a spray.
-
+# Changelog for `hspray`
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to the
+[Haskell Package Versioning Policy](https://pvp.haskell.org/).
+
+
+## 0.1.0.0 - 2022-12-11
+
+First release.
+
+
+## 0.1.1.0 - 2022-12-12
+
+* New functions `toList`, `sprayTerms` and `bombieriSpray`.
+
+* New operation `.^`, to multiply a spray by an integer.
+
+* Added some unit tests.
+
+
+## 0.1.2.0 - 2023-02-24
+
+New function `derivSpray`, to differentiate a spray.
+
+
+## 0.1.3.0 - 2023-08-29
+
+* `Powers(..)` is now exported.
+
+* Completed the README to show how to deal with symbolic coefficients.
+
diff --git a/LICENSE b/LICENSE
--- a/LICENSE
+++ b/LICENSE
@@ -1,674 +1,674 @@
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-                     END OF TERMS AND CONDITIONS
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-            How to Apply These Terms to Your New Programs
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-The hypothetical commands `show w' and `show c' should show the appropriate
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-  You should also get your employer (if you work as a programmer) or school,
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-For more information on this, and how to apply and follow the GNU GPL, see
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-
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+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
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+
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+    Copyright (C) <year>  <name of author>
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+    it under the terms of the GNU General Public License as published by
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+
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+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
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+
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+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
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+Public License instead of this License.  But first, please read
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diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -1,74 +1,130 @@
-# hspray
-
-<!-- badges: start -->
-[![Stack-lts](https://github.com/stla/hspray/actions/workflows/Stack-lts.yml/badge.svg)](https://github.com/stla/hspray/actions/workflows/Stack-lts.yml)
-[![Stack-nightly](https://github.com/stla/hspray/actions/workflows/Stack-nightly.yml/badge.svg)](https://github.com/stla/hspray/actions/workflows/Stack-nightly.yml)
-<!-- badges: end -->
-
-Simple multivariate polynomials in Haskell.
-
-___
-
-
-```haskell
-import Math.Algebra.Hspray
-x = lone 1 :: Spray Double
-y = lone 2 :: Spray Double
-z = lone 3 :: Spray Double
-poly = (2 *^ (x^**^3 ^*^ y ^*^ z) ^+^ x^**^2) ^*^ (4 *^ (x ^*^ y ^*^ z))
-prettySpray show "X" poly
--- "(4.0) * X^(3, 1, 1) + (8.0) * X^(4, 2, 2)"
-```
-
-More generally, one can use the type `Spray a` as long as the type `a` has 
-the instances `Eq` and `Algebra.Ring` (defined in the **numeric-prelude** 
-library). For example `a = Rational`:
-
-```haskell
-import Math.Algebra.Hspray
-import Data.Ratio
-x = lone 1 :: Spray Rational
-y = lone 2 :: Spray Rational
-z = lone 3 :: Spray Rational
-poly = ((2%3) *^ (x^**^3 ^*^ y ^*^ z) ^+^ x^**^2) ^*^ ((7%4) *^ (x ^*^ y ^*^ z))
-prettySpray show "X" poly
--- "(7 % 4) * X^(3, 1, 1) + (7 % 6) * X^(4, 2, 2)"
-```
-
-Or `a = Spray Double`:
-
-```haskell
-import Math.Algebra.Hspray
-p = lone 1 :: Spray Double
-x = lone 1 :: Spray (Spray Double)
-y = lone 2 :: Spray (Spray Double)
-poly = ((p *^ x) ^+^ (p *^ y))^**^2  
-prettySpray (prettySpray show "a") "X" poly
--- "((1.0) * a^(2)) * X^(0, 2) + ((2.0) * a^(2)) * X^(1, 1) + ((1.0) * a^(2)) * X^(2)"
-```
-
-Evaluation:
-
-```haskell
-import Math.Algebra.Hspray
-x = lone 1 :: Spray Double
-y = lone 2 :: Spray Double
-z = lone 3 :: Spray Double
-poly = 2 *^ (x ^*^ y ^*^ z) 
--- evaluate poly at x=2, y=1, z=2
-evalSpray poly [2, 1, 2]
--- 8.0
-```
-
-Differentiation:
-
-```haskell
-import Math.Algebra.Hspray
-x = lone 1 :: Spray Double
-y = lone 2 :: Spray Double
-z = lone 3 :: Spray Double
-poly = 2 *^ (x ^*^ y ^*^ z) ^+^ (3 *^ x^**^2)
--- derivate with respect to x
-prettySpray show "X" $ derivSpray 1 poly
--- "(2.0) * X^(0, 1, 1) + (6.0) * X^(1)"
-```
+# hspray
+
+<!-- badges: start -->
+[![Stack-lts](https://github.com/stla/hspray/actions/workflows/Stack-lts.yml/badge.svg)](https://github.com/stla/hspray/actions/workflows/Stack-lts.yml)
+[![Stack-nightly](https://github.com/stla/hspray/actions/workflows/Stack-nightly.yml/badge.svg)](https://github.com/stla/hspray/actions/workflows/Stack-nightly.yml)
+<!-- badges: end -->
+
+Simple multivariate polynomials in Haskell.
+
+___
+
+
+```haskell
+import Math.Algebra.Hspray
+x = lone 1 :: Spray Double
+y = lone 2 :: Spray Double
+z = lone 3 :: Spray Double
+poly = (2 *^ (x^**^3 ^*^ y ^*^ z) ^+^ x^**^2) ^*^ (4 *^ (x ^*^ y ^*^ z))
+prettySpray show "X" poly
+-- "(4.0) * X^(3, 1, 1) + (8.0) * X^(4, 2, 2)"
+```
+
+More generally, one can use the type `Spray a` as long as the type `a` has 
+the instances `Eq` and `Algebra.Ring` (defined in the **numeric-prelude** 
+library). For example `a = Rational`:
+
+```haskell
+import Math.Algebra.Hspray
+import Data.Ratio
+x = lone 1 :: Spray Rational
+y = lone 2 :: Spray Rational
+z = lone 3 :: Spray Rational
+poly = ((2%3) *^ (x^**^3 ^*^ y ^*^ z) ^+^ x^**^2) ^*^ ((7%4) *^ (x ^*^ y ^*^ z))
+prettySpray show "X" poly
+-- "(7 % 4) * X^(3, 1, 1) + (7 % 6) * X^(4, 2, 2)"
+```
+
+Or `a = Spray Double`:
+
+```haskell
+import Math.Algebra.Hspray
+p = lone 1 :: Spray Double
+x = lone 1 :: Spray (Spray Double)
+y = lone 2 :: Spray (Spray Double)
+poly = ((p *^ x) ^+^ (p *^ y))^**^2  
+prettySpray (prettySpray show "a") "X" poly
+-- "((1.0) * a^(2)) * X^(0, 2) + ((2.0) * a^(2)) * X^(1, 1) + ((1.0) * a^(2)) * X^(2)"
+```
+
+Evaluation:
+
+```haskell
+import Math.Algebra.Hspray
+x = lone 1 :: Spray Double
+y = lone 2 :: Spray Double
+z = lone 3 :: Spray Double
+poly = 2 *^ (x ^*^ y ^*^ z) 
+-- evaluate poly at x=2, y=1, z=2
+evalSpray poly [2, 1, 2]
+-- 8.0
+```
+
+Differentiation:
+
+```haskell
+import Math.Algebra.Hspray
+x = lone 1 :: Spray Double
+y = lone 2 :: Spray Double
+z = lone 3 :: Spray Double
+poly = 2 *^ (x ^*^ y ^*^ z) ^+^ (3 *^ x^**^2)
+-- derivate with respect to x
+prettySpray show "X" $ derivSpray 1 poly
+-- "(2.0) * X^(0, 1, 1) + (6.0) * X^(1)"
+```
+
+## Easier usage 
+
+To construct a polynomial using the ordinary symbols `+`, `*` and `-`, 
+one can hide these operators from **Prelude** and import them from 
+the **numeric-prelude** library:
+
+```haskell
+import Prelude hiding ((*), (+), (-))
+import qualified Prelude as P
+import Algebra.Additive              
+import Algebra.Module                
+import Algebra.Ring                  
+import Math.Algebra.Hspray
+```
+
+Or, maybe better (I didn't try yet), follow the "Usage" section on the 
+[Hackage page](https://hackage.haskell.org/package/numeric-prelude-0.4.4#usage) 
+of **numeric-prelude**.
+
+## Symbolic coefficients
+
+Assume you have the polynomial `a * (x² + y²) + 2b/3 * z`, 
+where `a` and `b` are symbolic coefficients. 
+You can define this polynomial as a `Spray` as follows:
+
+```haskell
+import Prelude hiding ((*), (+), (-))
+import qualified Prelude as P
+import Algebra.Additive              
+import Algebra.Module                
+import Algebra.Ring                  
+import Math.Algebra.Hspray
+import Data.Ratio
+
+x = lone 1 :: Spray (Spray Rational)
+y = lone 2 :: Spray (Spray Rational)
+z = lone 3 :: Spray (Spray Rational)
+a = lone 1 :: Spray Rational
+b = lone 2 :: Spray Rational
+
+poly = a *^ (x*x + y*y) + ((2%3) *^ b) *^ z 
+prettySpray (prettySpray show "a") "X" poly
+-- "((2 % 3) * a^(0, 1)) * X^(0, 0, 1) + ((1 % 1) * a^(1)) * X^(0, 2) + ((1 % 1) * a^(1)) * X^(2)"
+```
+
+The `prettySpray` function shows the expansion of the polynomial. 
+You can extract the powers and the coefficients as follows:
+
+```haskell
+l = toList poly
+map fst l
+-- [[0,0,1],[2],[0,2]]
+map toList $ map snd l
+-- [[([0,1],2 % 3)],[([1],1 % 1)],[([1],1 % 1)]]
+```
diff --git a/Setup.hs b/Setup.hs
--- a/Setup.hs
+++ b/Setup.hs
@@ -1,2 +1,2 @@
-import Distribution.Simple
-main = defaultMain
+import Distribution.Simple
+main = defaultMain
diff --git a/hspray.cabal b/hspray.cabal
--- a/hspray.cabal
+++ b/hspray.cabal
@@ -1,52 +1,53 @@
-name:                hspray
-version:             0.1.2.0
-synopsis:            Multivariate polynomials.
-description:         Manipulation of multivariate polynomials on a ring.
-homepage:            https://github.com/stla/hspray#readme
-license:             GPL-3
-license-file:        LICENSE
-author:              Stéphane Laurent
-maintainer:          laurent_step@outlook.fr
-copyright:           2022 Stéphane Laurent
-category:            Math, Algebra
-build-type:          Simple
-extra-source-files:  README.md
-                     CHANGELOG.md
-cabal-version:       >=1.10
-
-library
-  hs-source-dirs:      src
-  exposed-modules:     Math.Algebra.Hspray
-  build-depends:       base >= 4.7 && < 5
-                     , containers >= 0.6.4.1
-                     , hashable >= 1.3.4.0
-                     , unordered-containers >= 0.2.17.0
-                     , numeric-prelude >= 0.4.4
-                     , text >= 1.2.5.0
-  other-extensions:    FlexibleInstances
-                     , FlexibleContexts
-                     , MultiParamTypeClasses
-  default-language:    Haskell2010
-  ghc-options:         -Wall
-                       -Wcompat
-                       -Widentities
-                       -Wincomplete-record-updates
-                       -Wincomplete-uni-patterns
-                       -Wmissing-export-lists
-                       -Wmissing-home-modules
-                       -Wpartial-fields
-                       -Wredundant-constraints
-
-test-suite unit-tests
-  type:                 exitcode-stdio-1.0
-  main-is:              Main.hs
-  hs-source-dirs:       tests/
-  Build-Depends:        base >= 4.7 && < 5
-                      , tasty
-                      , tasty-hunit
-                      , hspray
-  Default-Language:     Haskell2010
-
-source-repository head
-  type:     git
-  location: https://github.com/stla/hspray
+name:                hspray
+version:             0.1.3.0
+synopsis:            Multivariate polynomials.
+description:         Manipulation of multivariate polynomials on a ring.
+homepage:            https://github.com/stla/hspray#readme
+license:             GPL-3
+license-file:        LICENSE
+author:              Stéphane Laurent
+maintainer:          laurent_step@outlook.fr
+copyright:           2022 Stéphane Laurent
+category:            Math, Algebra
+build-type:          Simple
+extra-source-files:  README.md
+                     CHANGELOG.md
+cabal-version:       >=1.10
+
+library
+  hs-source-dirs:      src
+  exposed-modules:     Math.Algebra.Hspray
+  build-depends:       base >= 4.7 && < 5
+                     , containers >= 0.6.4.1
+                     , hashable >= 1.3.4.0
+                     , unordered-containers >= 0.2.17.0
+                     , numeric-prelude >= 0.4.4
+                     , text >= 1.2.5.0
+  other-extensions:    FlexibleInstances
+                     , FlexibleContexts
+                     , MultiParamTypeClasses
+                     , InstanceSigs
+  default-language:    Haskell2010
+  ghc-options:         -Wall
+                       -Wcompat
+                       -Widentities
+                       -Wincomplete-record-updates
+                       -Wincomplete-uni-patterns
+                       -Wmissing-export-lists
+                       -Wmissing-home-modules
+                       -Wpartial-fields
+                       -Wredundant-constraints
+
+test-suite unit-tests
+  type:                 exitcode-stdio-1.0
+  main-is:              Main.hs
+  hs-source-dirs:       tests/
+  Build-Depends:        base >= 4.7 && < 5
+                      , tasty
+                      , tasty-hunit
+                      , hspray
+  Default-Language:     Haskell2010
+
+source-repository head
+  type:     git
+  location: https://github.com/stla/hspray
diff --git a/src/Math/Algebra/Hspray.hs b/src/Math/Algebra/Hspray.hs
--- a/src/Math/Algebra/Hspray.hs
+++ b/src/Math/Algebra/Hspray.hs
@@ -1,252 +1,256 @@
-{-# LANGUAGE FlexibleInstances #-}
-{-# LANGUAGE FlexibleContexts #-}
-{-# LANGUAGE MultiParamTypeClasses #-}
-
-module Math.Algebra.Hspray
-  ( Spray
-  , lone
-  , unitSpray
-  , constantSpray
-  , fromList
-  , toList
-  , sprayTerms
-  , (*^)
-  , (.^)
-  , (^+^)
-  , (^-^)
-  , (^*^)
-  , (^**^)
-  , evalSpray
-  , prettySpray
-  , composeSpray
-  , bombieriSpray
-  , derivSpray
-  ) where
-import qualified Algebra.Additive              as AlgAdd
-import qualified Algebra.Module                as AlgMod
-import qualified Algebra.Ring                  as AlgRing
-import qualified Data.Foldable                 as DF
-import           Data.Function                  ( on )
-import           Data.HashMap.Strict            ( HashMap )
-import qualified Data.HashMap.Strict           as HM
-import           Data.Hashable                  ( Hashable(hashWithSalt) )
-import           Data.List                      ( sortBy )
-import qualified Data.Sequence                 as S
-import           Data.Sequence                  ( (><)
-                                                , Seq
-                                                , dropWhileR
-                                                , (|>)
-                                                , index
-                                                , adjust
-                                                )
-import           Data.Text                      ( Text
-                                                , append
-                                                , cons
-                                                , intercalate
-                                                , pack
-                                                , snoc
-                                                , unpack
-                                                )
-
-
-infixr 7 *^, .^
-
-infixl 6 ^+^, ^-^
-
-infixl 7 ^*^
-
-infixr 8 ^**^
-
-
-data Powers = Powers
-  { exponents  :: Seq Int
-  , nvariables :: Int
-  }
-  deriving Show
-
-growSequence :: Seq Int -> Int -> Int -> Seq Int
-growSequence s m n = s >< t where t = S.replicate (n - m) 0
-
-harmonize :: (Powers, Powers) -> (Powers, Powers)
-harmonize (pows1, pows2) = (Powers e1' n, Powers e2' n)
- where
-  e1            = exponents pows1
-  e2            = exponents pows2
-  n1            = nvariables pows1
-  n2            = nvariables pows2
-  (e1', e2', n) = if n1 < n2
-    then (growSequence e1 n1 n2, e2, n2)
-    else (e1, growSequence e2 n2 n1, n1)
-
-instance Eq Powers where
-  pows1 == pows2 = exponents pows1' == exponents pows2'
-    where (pows1', pows2') = harmonize (pows1, pows2)
-
-instance Hashable Powers where
-  hashWithSalt k pows = hashWithSalt k (exponents pows, nvariables pows)
-
-type Spray a = HashMap Powers a
-
-instance (AlgAdd.C a, Eq a) => AlgAdd.C (Spray a) where
-  p + q = addSprays p q
-  zero   = HM.empty
-  negate = negateSpray
-
-instance (AlgRing.C a, Eq a) => AlgMod.C a (Spray a) where
-  lambda *> p = scaleSpray lambda p
-
-instance (AlgRing.C a, Eq a) => AlgRing.C (Spray a) where
-  p * q = multSprays p q
-  one = lone 0
-
--- | Addition of two sprays
-(^+^) :: (AlgAdd.C a, Eq a) => Spray a -> Spray a -> Spray a
-(^+^) p q = p AlgAdd.+ q
-
--- | Substraction of two sprays
-(^-^) :: (AlgAdd.C a, Eq a) => Spray a -> Spray a -> Spray a
-(^-^) p q = p AlgAdd.- q
-
--- | Multiply two sprays
-(^*^) :: (AlgRing.C a, Eq a) => Spray a -> Spray a -> Spray a
-(^*^) p q = p AlgRing.* q
-
--- | Power of a spray
-(^**^) :: (AlgRing.C a, Eq a) => Spray a -> Int -> Spray a
-(^**^) p n = AlgRing.product (replicate n p)
-
--- | Scale spray by a scalar
-(*^) :: (AlgRing.C a, Eq a) => a -> Spray a -> Spray a
-(*^) lambda pol = lambda AlgMod.*> pol
-
--- | Scale spray by an integer
-(.^) :: (AlgAdd.C a, Eq a) => Int -> Spray a -> Spray a
-(.^) k pol = if k >= 0
-  then AlgAdd.sum (replicate k pol)
-  else AlgAdd.negate $ AlgAdd.sum (replicate (-k) pol)
-
-simplifyPowers :: Powers -> Powers
-simplifyPowers pows = Powers s (S.length s)
-  where s = dropWhileR (== 0) (exponents pows)
-
-simplifySpray :: Spray a -> Spray a
-simplifySpray = HM.mapKeys simplifyPowers
-
-cleanSpray :: (AlgAdd.C a, Eq a) => Spray a -> Spray a
-cleanSpray p = HM.filter (/= AlgAdd.zero) (simplifySpray p)
-
-addSprays :: (AlgAdd.C a, Eq a) => Spray a -> Spray a -> Spray a
-addSprays p q = cleanSpray $ HM.foldlWithKey' f p q
-  where f s powers coef = HM.insertWith (AlgAdd.+) powers coef s
-
-negateSpray :: AlgAdd.C a => Spray a -> Spray a
-negateSpray = HM.map AlgAdd.negate
-
-scaleSpray :: (AlgRing.C a, Eq a) => a -> Spray a -> Spray a
-scaleSpray lambda p = cleanSpray $ HM.map (lambda AlgRing.*) p
-
-derivMonomial :: AlgRing.C a => Int -> (Powers, a) -> (Powers, a) 
-derivMonomial i (pows, coef) = if i' >= S.length expts 
-  then (Powers S.empty 0, AlgAdd.zero)
-  else (pows', coef')
-   where
-    i'     = i - 1
-    expts  = exponents pows
-    expt_i = expts `index` i'
-    expts' = adjust (subtract 1) i' expts
-    coef'  = AlgAdd.sum (replicate expt_i coef)
-    pows'  = Powers expts' (nvariables pows) 
-
-derivSpray :: (AlgRing.C a, Eq a) => Int -> Spray a -> Spray a
-derivSpray i p = cleanSpray $ HM.fromListWith (AlgAdd.+) monomials
- where
-  p'        = HM.toList p
-  monomials = [ derivMonomial i mp | mp <- p' ]
-
-
-multMonomial :: AlgRing.C a => (Powers, a) -> (Powers, a) -> (Powers, a)
-multMonomial (pows1, coef1) (pows2, coef2) = (pows, coef1 AlgRing.* coef2)
- where
-  (pows1', pows2') = harmonize (pows1, pows2)
-  expts            = S.zipWith (+) (exponents pows1') (exponents pows2')
-  pows             = Powers expts (nvariables pows1')
-
-multSprays :: (AlgRing.C a, Eq a) => Spray a -> Spray a -> Spray a
-multSprays p q = cleanSpray $ HM.fromListWith (AlgAdd.+) prods
- where
-  p'    = HM.toList p
-  q'    = HM.toList q
-  prods = [ multMonomial mp mq | mp <- p', mq <- q' ]
-
--- | Spray corresponding to polynomial x_n
-lone :: AlgRing.C a => Int -> Spray a
-lone n = HM.singleton pows AlgRing.one
- where
-  pows = if n == 0
-    then Powers S.empty 0
-    else Powers (S.replicate (n - 1) AlgAdd.zero |> AlgRing.one) n
-
--- | Unit spray
-unitSpray :: AlgRing.C a => Spray a
-unitSpray = lone 0
-
--- | Constant spray
-constantSpray :: (AlgRing.C a, Eq a) => a -> Spray a
-constantSpray c = c *^ lone 0
-
-evalMonomial :: AlgRing.C a => [a] -> (Powers, a) -> a
-evalMonomial xyz (powers, coeff) = coeff
-  AlgRing.* AlgRing.product (zipWith (AlgRing.^) xyz pows)
-  where pows = DF.toList (fromIntegral <$> exponents powers)
-
--- | Evaluate a spray
-evalSpray :: AlgRing.C a => Spray a -> [a] -> a
-evalSpray p xyz = AlgAdd.sum $ map (evalMonomial xyz) (HM.toList p)
-
-identify :: (AlgRing.C a, Eq a) => Spray a -> Spray (Spray a)
-identify = HM.map constantSpray
-
--- | Compose a spray with a change of variables
-composeSpray :: (AlgRing.C a, Eq a) => Spray a -> [Spray a] -> Spray a
-composeSpray p = evalSpray (identify p)
-
--- | Create a spray from list of terms
-fromList :: (AlgRing.C a, Eq a) => [([Int], a)] -> Spray a
-fromList x = cleanSpray $ HM.fromList $ map
-  (\(expts, coef) -> (Powers (S.fromList expts) (length expts), coef)) x
-
-prettyPowers :: String -> [Int] -> Text
-prettyPowers var pows = append (pack x) (cons '(' $ snoc string ')')
- where
-  x      = " " ++ var ++ "^"
-  string = intercalate (pack ", ") (map (pack . show) pows)
-
--- | Pretty form of a spray
-prettySpray :: (a -> String) -> String -> Spray a -> String
-prettySpray prettyCoef var p = unpack $ intercalate (pack " + ") stringTerms
- where
-  stringTerms = map stringTerm (sortBy (compare `on` fexpts) (HM.toList p))
-  fexpts term = exponents $ fst term
-  stringTerm term = append
-    (snoc (snoc (cons '(' $ snoc stringCoef ')') ' ') '*')
-    (prettyPowers var pows)
-   where
-    pows       = DF.toList $ exponents (fst term)
-    stringCoef = pack $ prettyCoef (snd term)
-
--- | Terms of a spray
-sprayTerms :: Spray a -> HashMap (Seq Int) a
-sprayTerms = HM.mapKeys exponents
-
--- | Spray as list
-toList :: Spray a -> [([Int], a)]
-toList p = HM.toList $ HM.mapKeys (DF.toList . exponents) p
-
--- | Bombieri spray
-bombieriSpray :: AlgAdd.C a => Spray a -> Spray a
-bombieriSpray = HM.mapWithKey f
- where
-  f pows          = times (pfactorial $ exponents pows)
-  pfactorial pows = product $ DF.toList $ factorial <$> S.filter (/= 0) pows
-  factorial n     = product [1 .. n]
-  times k x       = AlgAdd.sum (replicate k x)
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE InstanceSigs #-}
+
+module Math.Algebra.Hspray
+  ( Powers (..)
+  , Spray
+  , lone
+  , unitSpray
+  , constantSpray
+  , fromList
+  , toList
+  , sprayTerms
+  , (*^)
+  , (.^)
+  , (^+^)
+  , (^-^)
+  , (^*^)
+  , (^**^)
+  , evalSpray
+  , prettySpray
+  , composeSpray
+  , bombieriSpray
+  , derivSpray
+  ) where
+import qualified Algebra.Additive              as AlgAdd
+import qualified Algebra.Module                as AlgMod
+import qualified Algebra.Ring                  as AlgRing
+import qualified Data.Foldable                 as DF
+import           Data.Function                  ( on )
+import           Data.HashMap.Strict            ( HashMap )
+import qualified Data.HashMap.Strict           as HM
+import           Data.Hashable                  ( Hashable(hashWithSalt) )
+import           Data.List                      ( sortBy )
+import qualified Data.Sequence                 as S
+import           Data.Sequence                  ( (><)
+                                                , Seq
+                                                , dropWhileR
+                                                , (|>)
+                                                , index
+                                                , adjust
+                                                )
+import           Data.Text                      ( Text
+                                                , append
+                                                , cons
+                                                , intercalate
+                                                , pack
+                                                , snoc
+                                                , unpack
+                                                )
+
+
+infixr 7 *^, .^
+
+infixl 6 ^+^, ^-^
+
+infixl 7 ^*^
+
+infixr 8 ^**^
+
+
+data Powers = Powers
+  { exponents  :: Seq Int
+  , nvariables :: Int
+  }
+  deriving Show
+
+growSequence :: Seq Int -> Int -> Int -> Seq Int
+growSequence s m n = s >< t where t = S.replicate (n - m) 0
+
+harmonize :: (Powers, Powers) -> (Powers, Powers)
+harmonize (pows1, pows2) = (Powers e1' n, Powers e2' n)
+ where
+  e1            = exponents pows1
+  e2            = exponents pows2
+  n1            = nvariables pows1
+  n2            = nvariables pows2
+  (e1', e2', n) = if n1 < n2
+    then (growSequence e1 n1 n2, e2, n2)
+    else (e1, growSequence e2 n2 n1, n1)
+
+instance Eq Powers where
+  (==) :: Powers -> Powers -> Bool
+  pows1 == pows2 = exponents pows1' == exponents pows2'
+    where (pows1', pows2') = harmonize (pows1, pows2)
+
+instance Hashable Powers where
+  hashWithSalt :: Int -> Powers -> Int
+  hashWithSalt k pows = hashWithSalt k (exponents pows, nvariables pows)
+
+type Spray a = HashMap Powers a
+
+instance (AlgAdd.C a, Eq a) => AlgAdd.C (Spray a) where
+  p + q = addSprays p q
+  zero   = HM.empty
+  negate = negateSpray
+
+instance (AlgRing.C a, Eq a) => AlgMod.C a (Spray a) where
+  lambda *> p = scaleSpray lambda p
+
+instance (AlgRing.C a, Eq a) => AlgRing.C (Spray a) where
+  p * q = multSprays p q
+  one = lone 0
+
+-- | Addition of two sprays
+(^+^) :: (AlgAdd.C a, Eq a) => Spray a -> Spray a -> Spray a
+(^+^) p q = p AlgAdd.+ q
+
+-- | Substraction of two sprays
+(^-^) :: (AlgAdd.C a, Eq a) => Spray a -> Spray a -> Spray a
+(^-^) p q = p AlgAdd.- q
+
+-- | Multiply two sprays
+(^*^) :: (AlgRing.C a, Eq a) => Spray a -> Spray a -> Spray a
+(^*^) p q = p AlgRing.* q
+
+-- | Power of a spray
+(^**^) :: (AlgRing.C a, Eq a) => Spray a -> Int -> Spray a
+(^**^) p n = AlgRing.product (replicate n p)
+
+-- | Scale spray by a scalar
+(*^) :: (AlgRing.C a, Eq a) => a -> Spray a -> Spray a
+(*^) lambda pol = lambda AlgMod.*> pol
+
+-- | Scale spray by an integer
+(.^) :: (AlgAdd.C a, Eq a) => Int -> Spray a -> Spray a
+(.^) k pol = if k >= 0
+  then AlgAdd.sum (replicate k pol)
+  else AlgAdd.negate $ AlgAdd.sum (replicate (-k) pol)
+
+simplifyPowers :: Powers -> Powers
+simplifyPowers pows = Powers s (S.length s)
+  where s = dropWhileR (== 0) (exponents pows)
+
+simplifySpray :: Spray a -> Spray a
+simplifySpray = HM.mapKeys simplifyPowers
+
+cleanSpray :: (AlgAdd.C a, Eq a) => Spray a -> Spray a
+cleanSpray p = HM.filter (/= AlgAdd.zero) (simplifySpray p)
+
+addSprays :: (AlgAdd.C a, Eq a) => Spray a -> Spray a -> Spray a
+addSprays p q = cleanSpray $ HM.foldlWithKey' f p q
+  where f s powers coef = HM.insertWith (AlgAdd.+) powers coef s
+
+negateSpray :: AlgAdd.C a => Spray a -> Spray a
+negateSpray = HM.map AlgAdd.negate
+
+scaleSpray :: (AlgRing.C a, Eq a) => a -> Spray a -> Spray a
+scaleSpray lambda p = cleanSpray $ HM.map (lambda AlgRing.*) p
+
+derivMonomial :: AlgRing.C a => Int -> (Powers, a) -> (Powers, a) 
+derivMonomial i (pows, coef) = if i' >= S.length expts 
+  then (Powers S.empty 0, AlgAdd.zero)
+  else (pows', coef')
+   where
+    i'     = i - 1
+    expts  = exponents pows
+    expt_i = expts `index` i'
+    expts' = adjust (subtract 1) i' expts
+    coef'  = AlgAdd.sum (replicate expt_i coef)
+    pows'  = Powers expts' (nvariables pows) 
+
+derivSpray :: (AlgRing.C a, Eq a) => Int -> Spray a -> Spray a
+derivSpray i p = cleanSpray $ HM.fromListWith (AlgAdd.+) monomials
+ where
+  p'        = HM.toList p
+  monomials = [ derivMonomial i mp | mp <- p' ]
+
+
+multMonomial :: AlgRing.C a => (Powers, a) -> (Powers, a) -> (Powers, a)
+multMonomial (pows1, coef1) (pows2, coef2) = (pows, coef1 AlgRing.* coef2)
+ where
+  (pows1', pows2') = harmonize (pows1, pows2)
+  expts            = S.zipWith (+) (exponents pows1') (exponents pows2')
+  pows             = Powers expts (nvariables pows1')
+
+multSprays :: (AlgRing.C a, Eq a) => Spray a -> Spray a -> Spray a
+multSprays p q = cleanSpray $ HM.fromListWith (AlgAdd.+) prods
+ where
+  p'    = HM.toList p
+  q'    = HM.toList q
+  prods = [ multMonomial mp mq | mp <- p', mq <- q' ]
+
+-- | Spray corresponding to polynomial x_n
+lone :: AlgRing.C a => Int -> Spray a
+lone n = HM.singleton pows AlgRing.one
+ where
+  pows = if n == 0
+    then Powers S.empty 0
+    else Powers (S.replicate (n - 1) AlgAdd.zero |> AlgRing.one) n
+
+-- | Unit spray
+unitSpray :: AlgRing.C a => Spray a
+unitSpray = lone 0
+
+-- | Constant spray
+constantSpray :: (AlgRing.C a, Eq a) => a -> Spray a
+constantSpray c = c *^ lone 0
+
+evalMonomial :: AlgRing.C a => [a] -> (Powers, a) -> a
+evalMonomial xyz (powers, coeff) = coeff
+  AlgRing.* AlgRing.product (zipWith (AlgRing.^) xyz pows)
+  where pows = DF.toList (fromIntegral <$> exponents powers)
+
+-- | Evaluate a spray
+evalSpray :: AlgRing.C a => Spray a -> [a] -> a
+evalSpray p xyz = AlgAdd.sum $ map (evalMonomial xyz) (HM.toList p)
+
+identify :: (AlgRing.C a, Eq a) => Spray a -> Spray (Spray a)
+identify = HM.map constantSpray
+
+-- | Compose a spray with a change of variables
+composeSpray :: (AlgRing.C a, Eq a) => Spray a -> [Spray a] -> Spray a
+composeSpray p = evalSpray (identify p)
+
+-- | Create a spray from list of terms
+fromList :: (AlgRing.C a, Eq a) => [([Int], a)] -> Spray a
+fromList x = cleanSpray $ HM.fromList $ map
+  (\(expts, coef) -> (Powers (S.fromList expts) (length expts), coef)) x
+
+prettyPowers :: String -> [Int] -> Text
+prettyPowers var pows = append (pack x) (cons '(' $ snoc string ')')
+ where
+  x      = " " ++ var ++ "^"
+  string = intercalate (pack ", ") (map (pack . show) pows)
+
+-- | Pretty form of a spray
+prettySpray :: (a -> String) -> String -> Spray a -> String
+prettySpray prettyCoef var p = unpack $ intercalate (pack " + ") stringTerms
+ where
+  stringTerms = map stringTerm (sortBy (compare `on` fexpts) (HM.toList p))
+  fexpts term = exponents $ fst term
+  stringTerm term = append
+    (snoc (snoc (cons '(' $ snoc stringCoef ')') ' ') '*')
+    (prettyPowers var pows)
+   where
+    pows       = DF.toList $ exponents (fst term)
+    stringCoef = pack $ prettyCoef (snd term)
+
+-- | Terms of a spray
+sprayTerms :: Spray a -> HashMap (Seq Int) a
+sprayTerms = HM.mapKeys exponents
+
+-- | Spray as list
+toList :: Spray a -> [([Int], a)]
+toList p = HM.toList $ HM.mapKeys (DF.toList . exponents) p
+
+-- | Bombieri spray
+bombieriSpray :: AlgAdd.C a => Spray a -> Spray a
+bombieriSpray = HM.mapWithKey f
+ where
+  f pows          = times (pfactorial $ exponents pows)
+  pfactorial pows = product $ DF.toList $ factorial <$> S.filter (/= 0) pows
+  factorial n     = product [1 .. n]
+  times k x       = AlgAdd.sum (replicate k x)
diff --git a/tests/Main.hs b/tests/Main.hs
--- a/tests/Main.hs
+++ b/tests/Main.hs
@@ -1,73 +1,73 @@
-module Main where
-import           Data.Ratio                     ( (%) )
-import           Math.Algebra.Hspray            ( Spray,
-                                                  (^+^),
-                                                  (^*^),
-                                                  (^**^),
-                                                  (*^),
-                                                  lone,
-                                                  unitSpray,
-                                                  evalSpray,
-                                                  composeSpray,
-                                                  fromList,
-                                                  toList,
-                                                  bombieriSpray,
-                                                  derivSpray )
-import           Test.Tasty                     ( defaultMain
-                                                , testGroup
-                                                )
-import           Test.Tasty.HUnit               ( assertEqual
-                                                , testCase
-                                                )
-
-main :: IO ()
-main = defaultMain $ testGroup
-  "Testing hspray"
-
-  [ testCase "bombieriSpray" $ do
-      let
-        x = lone 1 :: Spray Rational
-        y = lone 2 :: Spray Rational
-        z = lone 3 :: Spray Rational
-        poly =
-          (2 % 1) *^ ((2 % 1) *^ (x ^**^ 3 ^*^ y ^**^ 2)) ^+^ (4 % 1) *^ z ^+^ (5 % 1) *^ unitSpray
-        bpoly =
-          (24 % 1) *^ ((2 % 1) *^ (x ^**^ 3 ^*^ y ^**^ 2)) ^+^ (4 % 1) *^ z ^+^ (5 % 1) *^ unitSpray
-      assertEqual "" bpoly (bombieriSpray poly),
-
-    testCase "composeSpray" $ do
-      let
-        x = lone 1 :: Spray Int
-        y = lone 2 :: Spray Int
-        z = lone 3 :: Spray Int
-        p = 2 *^ (2 *^ (x ^**^ 3 ^*^ y ^**^ 2)) ^+^ 4 *^ z ^+^ 5 *^ unitSpray
-        px = x ^+^ y ^+^ z
-        py = x ^*^ y ^*^ z
-        pz = y ^**^ 2
-        q = composeSpray p [px, py, pz]
-        xyz = [2, 3, 4]
-        pxyz = map (`evalSpray` xyz) [px, py, pz]
-      assertEqual "" (evalSpray p pxyz) (evalSpray q xyz),
-
-    testCase "fromList . toList = identity" $ do
-      let
-        x = lone 1 :: Spray Int
-        y = lone 2 :: Spray Int
-        z = lone 3 :: Spray Int
-        p = 2 *^ (2 *^ (x ^**^ 3 ^*^ y ^**^ 2)) ^+^ 4 *^ z ^+^ 5 *^ unitSpray
-      assertEqual "" p (fromList . toList $ p),
-
-    testCase "derivSpray" $ do
-      let
-        x = lone 1 :: Spray Int
-        y = lone 2 :: Spray Int
-        z = lone 3 :: Spray Int
-        p1 = x ^+^ y ^*^ z ^**^ 3
-        p2 = (x ^*^ y ^*^ z) ^+^ (2 *^ (x ^**^ 3 ^*^ y ^**^ 2))
-        q = p1 ^*^ p2
-        p1' = derivSpray 1 p1
-        p2' = derivSpray 1 p2
-        q'  = derivSpray 1 q
-      assertEqual "" q' ((p1' ^*^ p2) ^+^ (p1 ^*^ p2'))
-
-  ]
+module Main where
+import           Data.Ratio                     ( (%) )
+import           Math.Algebra.Hspray            ( Spray,
+                                                  (^+^),
+                                                  (^*^),
+                                                  (^**^),
+                                                  (*^),
+                                                  lone,
+                                                  unitSpray,
+                                                  evalSpray,
+                                                  composeSpray,
+                                                  fromList,
+                                                  toList,
+                                                  bombieriSpray,
+                                                  derivSpray )
+import           Test.Tasty                     ( defaultMain
+                                                , testGroup
+                                                )
+import           Test.Tasty.HUnit               ( assertEqual
+                                                , testCase
+                                                )
+
+main :: IO ()
+main = defaultMain $ testGroup
+  "Testing hspray"
+
+  [ testCase "bombieriSpray" $ do
+      let
+        x = lone 1 :: Spray Rational
+        y = lone 2 :: Spray Rational
+        z = lone 3 :: Spray Rational
+        poly =
+          (2 % 1) *^ ((2 % 1) *^ (x ^**^ 3 ^*^ y ^**^ 2)) ^+^ (4 % 1) *^ z ^+^ (5 % 1) *^ unitSpray
+        bpoly =
+          (24 % 1) *^ ((2 % 1) *^ (x ^**^ 3 ^*^ y ^**^ 2)) ^+^ (4 % 1) *^ z ^+^ (5 % 1) *^ unitSpray
+      assertEqual "" bpoly (bombieriSpray poly),
+
+    testCase "composeSpray" $ do
+      let
+        x = lone 1 :: Spray Int
+        y = lone 2 :: Spray Int
+        z = lone 3 :: Spray Int
+        p = 2 *^ (2 *^ (x ^**^ 3 ^*^ y ^**^ 2)) ^+^ 4 *^ z ^+^ 5 *^ unitSpray
+        px = x ^+^ y ^+^ z
+        py = x ^*^ y ^*^ z
+        pz = y ^**^ 2
+        q = composeSpray p [px, py, pz]
+        xyz = [2, 3, 4]
+        pxyz = map (`evalSpray` xyz) [px, py, pz]
+      assertEqual "" (evalSpray p pxyz) (evalSpray q xyz),
+
+    testCase "fromList . toList = identity" $ do
+      let
+        x = lone 1 :: Spray Int
+        y = lone 2 :: Spray Int
+        z = lone 3 :: Spray Int
+        p = 2 *^ (2 *^ (x ^**^ 3 ^*^ y ^**^ 2)) ^+^ 4 *^ z ^+^ 5 *^ unitSpray
+      assertEqual "" p (fromList . toList $ p),
+
+    testCase "derivSpray" $ do
+      let
+        x = lone 1 :: Spray Int
+        y = lone 2 :: Spray Int
+        z = lone 3 :: Spray Int
+        p1 = x ^+^ y ^*^ z ^**^ 3
+        p2 = (x ^*^ y ^*^ z) ^+^ (2 *^ (x ^**^ 3 ^*^ y ^**^ 2))
+        q = p1 ^*^ p2
+        p1' = derivSpray 1 p1
+        p2' = derivSpray 1 p2
+        q'  = derivSpray 1 q
+      assertEqual "" q' ((p1' ^*^ p2) ^+^ (p1 ^*^ p2'))
+
+  ]
