diff --git a/python/fileformat.py b/python/fileformat.py
index 9361466..2906f98 100644
--- a/python/fileformat.py
+++ b/python/fileformat.py
@@ -1,5 +1,10 @@
 import types
 
+# The possible fields for each type of object.
+#
+# Each field is a tuple (fieldname, fieldtype).  Fieldtype is either
+# 's' (string), 'd' (dictionary, where the values are lists of
+# strings), 'l' (list of strings).
 fields = {
     'book':
         [('isbn', 's'), ('title', 's'), ('category', 's'),
@@ -11,6 +16,12 @@ fields = {
         [('id', 's'), ('first_name', 's'), ('last_name', 's')],
     'category':
         [('id', 's'), ('name', 's'), ('placement', 'l')] }
+
+# Fields associated with each action.
+#
+# The 'type' is a key in the fields dictionary, indicating the set of
+# fields which can be used with the action.  The 'required' list is a
+# list of fields which must be present for the action to be accepted.
 action_fields = {
     'new-book':
         { 'type': 'book',
@@ -40,18 +51,25 @@ action_fields = {
         { 'type': 'category',
           'required': ['id'] } }
 
+class CommitFormatSyntaxError(Exception):
+    pass
+
 def read_action(lines):
     d = {}
     lastfield = None
     for line in lines:
         if len(line) == 0:
-            raise 'Empty line in action'
+            raise CommitFormatSyntaxError('Empty line in action')
         if line[0] in [' ', '\t']: # continuation line
             if not lastfield:
-                raise 'First line is continuation line'
+                raise CommitFormatSyntaxError('First line is continuation line: ' + line)
             d[lastfield] = d[lastfield] + '\n' + line.strip()
         else:
             field, value = line.split(':', 1)
+            # reader = { 's': read_field_value_str,
+            #            'd': read_field_value_dict,
+            #            'l': read_field_value_list }[
+            # translated_value = 
             # TODO skriv ferdig
 
 def write_field_value_str(val):