'Remove a flexible attribute from the model.'

def __delitem__(self, key):

    if (key in self._values_flex):
        del self._values_flex[key]
        self._dirty.add(key)
    elif (key in self._getters()):
        raise KeyError(u'computed    field    {0}    cannot    be    deleted'.format(key))
    elif (key in self._fields):
        raise KeyError(u'fixed    field    {0}    cannot    be    deleted'.format(key))
    else:
        raise KeyError(u'no    such    field    {0}'.format(key))
