diff --git a/swh/model/fields/simple.py b/swh/model/fields/simple.py
index f1bff95..d850285 100644
--- a/swh/model/fields/simple.py
+++ b/swh/model/fields/simple.py
@@ -1,71 +1,75 @@
+# Copyright (C) 2015  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
 
 import datetime
 import numbers
 
 from ..exceptions import ValidationError
 
 
 def validate_type(value, type):
     """Validate that value is an integer"""
     if not isinstance(value, type):
         if isinstance(type, tuple):
             typestr = 'one of %s' % ', '.join(typ.__name__ for typ in type)
         else:
             typestr = type.__name__
         raise ValidationError(
             'Unexpected type %(type)s, expected %(expected_type)s',
             params={
                 'type': value.__class__.__name__,
                 'expected_type': typestr,
             },
             code='unexpected-type'
         )
 
     return True
 
 
 def validate_int(value):
     """Validate that the given value is an int"""
     return validate_type(value, numbers.Integral)
 
 
 def validate_str(value):
     """Validate that the given value is a string"""
     return validate_type(value, str)
 
 
 def validate_datetime(value):
     """Validate that the given value is either a datetime, or a numeric number
     of seconds since the UNIX epoch."""
 
     errors = []
     try:
         validate_type(value, (datetime.datetime, numbers.Real))
     except ValidationError as e:
         errors.append(e)
 
     if isinstance(value, datetime.datetime) and value.tzinfo is None:
         errors.append(ValidationError(
             'Datetimes must be timezone-aware in swh',
             code='datetime-without-tzinfo',
         ))
 
     if errors:
         raise ValidationError(errors)
 
     return True
 
 
 def validate_enum(value, expected_values):
     """Validate that value is contained in expected_values"""
     if value not in expected_values:
         raise ValidationError(
             'Unexpected value %(value)s, expected one of %(expected_values)',
             params={
                 'value': value,
                 'expected_values': ', '.join(sorted(expected_values)),
             },
             code='unexpected-value',
         )
 
     return True