'Widget renderer for the input field. To be implemented in
subclass (if required) and to be set as widget=self for the
field returned by the resolve()-method of this form element.
@param field: the input field
@param value: the value to populate the widget
@param attributes: attributes for the widget
@return: the widget for this form element as HTML helper'

def __call__(self, field, value, **attributes):

    raise NotImplementedError
