minor: substatements function, comments

Logic/Statement/Laws.hs

@@ -41,6 +41,12 @@ laws =
   , mkLaw "iff_or"  "(A<->B)" "((A&B)|(!A&!B))"
   ]
 
+{-
+ghci> import Logic.Statement.Eval (bucket, Bucket(Tautology))
+ghci> all (== Tautology) $ map (\law -> bucket $ Iff (lawLhs law) (lawRhs law)) laws
+True
+-}
+
 lookupLaw :: String -> Maybe Law
 lookupLaw name = listToMaybe $ filter (\law -> lawName law == name) laws
 
@@ -48,7 +54,7 @@ match
   :: Statement
   -- ^ pattern
   -> Statement
-  -- ^ statement to search
+  -- ^ statement to search within
   -> Maybe [(String, Statement)]
   -- ^ mapping from pattern-statement atoms to search-statement parts
 match = aux []
@@ -104,19 +110,20 @@ Just [("C",Atom "r"),("B",Atom "q"),("A",Atom "p")]
 -}
 
 data SwapError
-  = Indeterminate
+  = IndeterminateSwap
   -- ^ An atom in p2 doesn't exist in p1.
   -- Strictly: an atom in p2 doesn't exist in the result from `match`
   -- (matters only if `match` is implemented incorrectly).
-  -- If for atoms we used terms of a type instead of strings,
-  -- we could avoid needing this error.
+  -- Theoretically if for atoms we used terms of a type instead of strings, we
+  -- could avoid needing this error, but I think we still wouldn't be able
+  -- to return a mapping from atom-type-1 to atom-type-2 in a type safe way.
   | NonMatchingPattern
   deriving Show
 
 swap :: Statement -> Statement -> Statement -> Either SwapError Statement
 swap p1 p2 s = do
   mapping <- maybe (Left NonMatchingPattern) Right $ match p1 s
-  maybe (Left Indeterminate) Right $ aux mapping p2
+  maybe (Left IndeterminateSwap) Right $ aux mapping p2
   where
     aux :: [(String, Statement)] -> Statement -> Maybe Statement
     aux mapping (Atom key) = lookup key mapping