[form] cache field value in form.formvalues using (field, form) key since in some case the same field instance may be shared accross multiple forms, which may share the formvalues dict in case of sub-forms
"""this contains the cube-specific entities' classes
:organization: Logilab
:copyright: 2001-2010 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.
:contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr
:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses
"""