Add types to Credit module and export

Credit.hs

 --
 -- orbit-int credits (for checking termination of orbit computation)
 --
-module Credit( credit
+module Credit( -- Types
+               ACredit
+             , Credit
+               -- Functions
+             , credit
              , credit_atomic
              , debit_atomic
              , debit_atomic_nz
@@ -20,9 +24,11 @@ module Credit( credit
 -- represented by the empty list. The maximally possible credit, 1,
 -- is represented by [0].
 
+type ACredit = Int
+type Credit = [ACredit]
 
 -- credit_atomic(K, C) adds the atomic credit 1/{2^K} to the credit C.
-credit_atomic :: Int -> [Int] -> [Int]
+credit_atomic :: Int -> Credit -> Credit
 credit_atomic k [] = [k]
 credit_atomic k (c : cs) | k > c  = k : c : cs
                          | k == c = credit_atomic (k - 1) cs
@@ -30,7 +36,7 @@ credit_atomic k (c : cs) | k > c  = k : c : cs
 
 -- credit(C1, C2) returns a list representing the sum of the credit
 -- represented by the lists C1 and C2.
-credit :: [Int] -> [Int] -> [Int]
+credit :: Credit -> Credit -> Credit
 credit c1 c2 = foldl (flip credit_atomic) c2 c1
 
 -- debit_atomic(C) returns a pair {K',C'} where K' is an integer
@@ -38,7 +44,7 @@ credit c1 c2 = foldl (flip credit_atomic) c2 c1
 -- some credit (which may be zero) such that the sum of the credits
 -- represented by K' and C' equals the credit represented by C.
 -- Precondition: C must represent non-zero credit.
-debit_atomic :: [Int] -> (Int, [Int])
+debit_atomic :: Credit -> (ACredit, Credit)
 debit_atomic (c : cs) = (c, cs) -- debit smallest unit of credit
 
 -- debit_atomic_nz(C) returns a pair {K',C'} where K' is an integer
@@ -46,25 +52,25 @@ debit_atomic (c : cs) = (c, cs) -- debit smallest unit of credit
 -- some non-zero credit such that the sum of the credits
 -- represented by K' and C' equals the credit represented by C.
 -- Precondition: C must represent non-zero credit.
-debit_atomic_nz :: [Int] -> (Int, [Int])
+debit_atomic_nz :: Credit -> (ACredit, Credit)
 debit_atomic_nz [c] = (c + 1, [c + 1]) -- debit half the credit
 debit_atomic_nz (c : cs) = (c, cs) -- debit smallest unit of credit;
 -- case only applies if Cs non-empty
 
 -- zero/0 produces zero credit.
-zero :: [Int]
+zero :: Credit
 zero = []
 
 -- one/0 produces credit one.
-one :: [Int]
+one :: Credit
 one = [0]
 
 -- is_zero/1 tests whether its argument represents zero credit.
-is_zero :: [Int] -> Bool
+is_zero :: Credit -> Bool
 is_zero [] = True
 is_zero _  = False
 
 -- is_one/1 tests whether its argument represents maximal credit 1.
-is_one :: [Int] -> Bool
+is_one :: Credit -> Bool
 is_one [0] = True
 is_one _   = False