def register_style( element )
      # get namespace prefix for this element
      style_name = element.attribute("#{element.prefix}:style-name");
      if (style_name != nil) then
        style_name = style_name.value.tr_s('.','_')
        if (@style_info[style_name] != nil) then
          @style_info[style_name].block_used = true
        end
      end
      return style_name
    end
def register_style( element )
      # get namespace prefix for this element
      style_name = element.attribute("#{element.prefix}:style-name");
      if (style_name != nil) then
        style_name = style_name.value.tr_s('.','_')
        if (@style_info[style_name] != nil) then
          @style_info[style_name].block_used = true
        end
      end
      return style_name
    end
Return the style name for this element, with periods
 changed to underscores to make it valid CSS.

 Side effect: registers this style as "having been used"
 in the document