Monitor webpage changes using google docs

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http://superuser.com – I want to monitor a web page.The best solution I found was google docs ( apart from feed43 which has limitations ). I'm using the import HTML function in google spreadsheet which will update the cell value whenever there is any change in data on that web page. However, I don't know how to save previous value somewhere and if possible append the new updated value to next row of spreadsheet instead of over writing the data on that cell. Any ideas?? (HowTos)