diff --git a/src/Rextra/Nfa.hs b/src/Rextra/Nfa.hs
index f26da9f..c9cbbf9 100644
--- a/src/Rextra/Nfa.hs
+++ b/src/Rextra/Nfa.hs
@@ -1,23 +1,53 @@
-module Rextra.Nfa where
--- TODO don't export internals
+module Rextra.Nfa (
+  -- * Nondeterministic Finite Automaton
+    Nfa
+  , State
+  -- ** Constructing
+  , nfa
+  , nfa'
+  -- ** Using
+  , nfaStates
+  , nfaEntryState
+  , nfaExitStates
+  , nfaTransition
+  -- ** Transitions
+  , TransitionCondition
+  , specialStates
+  , accepts
+  ) where
 
 import qualified Data.Map.Strict as Map
 import qualified Data.Set as Set
 
+-- | This condition determines which tokens a state transition applies to.
+--
+-- This representation is based on the assumption that there can be an
+-- infinite number of different tokens. The condition thus contains a
+-- default answer to the question "Does this transition apply to this
+-- token?", and all the exceptions for which the answer is negated.
 data TransitionCondition t
   = Only (Set.Set t)
   | AllExcept (Set.Set t)
 
+-- | The states which are treated differently from the default by the
+-- 'TransitionCondition'.
 specialStates :: TransitionCondition t -> Set.Set t
 specialStates (Only s)      = s
 specialStates (AllExcept s) = s
 
+-- | Whether the condition holds true for a token.
 accepts :: (Ord t) => TransitionCondition t -> t -> Bool
 accepts (Only s)      t = Set.member t s
 accepts (AllExcept s) t = Set.notMember t s
 
+-- | A state consists of the transitions to other states, and the
+-- conditions under which those transitions happen.
 type State s t = [(TransitionCondition t, s)]
 
+-- | A type representing a nondeterministic finite automaton.
+--
+-- It has one entry state and any number of exit states, which can be
+-- interpreted as accepting states when the NFA is run.
 data Nfa s t = Nfa
   { nfaStates     :: Map.Map s (State s t)
   , nfaEntryState :: s
@@ -25,10 +55,36 @@ data Nfa s t = Nfa
   }
 
 {-
- - Constructing and modifying a NFA
+ - Constructing a NFA
  -}
 
--- TODO
+integrityCheck :: (Ord s) => Nfa s t -> Bool
+integrityCheck nfa =
+  let referencedStates = Set.unions
+        [ Set.singleton (nfaEntryState nfa)
+        , nfaExitStates nfa
+        , Set.fromList . map snd . concat . Map.elems $ nfaStates nfa
+        ]
+  in  referencedStates `Set.isSubsetOf` Map.keysSet (nfaStates nfa)
+
+-- | Construct an 'Nfa' from all its components.
+--
+-- This constructor function performs some error checking required to
+-- keep the data structure internally consistent. At the moment, this
+-- is limited to checking whether all state names mentioned anywhere
+-- in the data struture actually exist in the state map.
+nfa :: (Ord s)
+    => Map.Map s (State s t) -- ^ The state lookup map (maps state name to state itself)
+    -> s                     -- ^ The entry state (starting state)
+    -> Set.Set s             -- ^ The exit states
+    -> Maybe (Nfa s t)       -- ^ The 'Nfa', if the data didn't show any inconsistencies
+nfa states entryState exitStates =
+  let myNfa = Nfa{nfaStates=states, nfaEntryState=entryState, nfaExitStates=exitStates}
+  in  if integrityCheck myNfa then Just myNfa else Nothing
+
+-- | A version of 'nfa' using argument formats that should be easier to work with.
+nfa' :: (Ord s) => [(s, State s t)] -> s -> [s] -> Maybe (Nfa s t)
+nfa' states entryState exitStates = nfa (Map.fromList states) entryState (Set.fromList exitStates)
 
 {-
  - "Executing" a NFA
@@ -37,9 +93,18 @@ data Nfa s t = Nfa
 getState :: (Ord s) => Nfa s t -> s -> State s t
 getState nfa s = nfaStates nfa Map.! s
 
--- * Starting from a state, find all the states that it can transition to with token 't'.
+-- | Starting from a state, find all the states that it can transition to with token @t@.
 nextStates :: (Ord s, Ord t) => State s t -> t -> Set.Set s
 nextStates state t = Set.fromList . map snd . filter (\(cond, _) -> cond `accepts` t) $ state
 
+-- | The NFA's transition function.
+--
+-- Since this is a /nondeterministic/ finite automaton, the transition
+-- function does not operate on individual states, but rather on a set
+-- of current states.
+--
+-- __Warning__: This function does /not/ check whether the states
+-- actually exist in the automaton, and it crashes if an invalid state
+-- is used.
 nfaTransition :: (Ord s, Ord t) => Nfa s t -> Set.Set s -> t -> Set.Set s
 nfaTransition nfa ss t = foldMap (\s -> nextStates (getState nfa s) t) ss