Christmas Bird Count history report system

The XHTML rendering will be a td element. It contains a non-breaking space so that in a ruled table it won't look like a blank spot. Additional CSS classes may be passed in through the addCss argument; see Section 59, “combineCss(): Combine CSS classes from different sources”.

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    def html(self, sox, addCss=None):
        '''Render a blank cell.
        '''
        sox.leaf("td", NON_BREAKING_SPACE,
                 combineCss(CLASS_SPACER_CELL, addCss))