1. A method of enabling backward navigation in a content space, said method comprising: receiving a first message digest that identifies a first computer file; receiving a second message digest that identifies a second computer file; receiving an indication that said second computer file is a later version of said first computer file; creating a descriptor file that includes said first message digest and said second message digest; and creating a third message digest of said descriptor file and returning said third message digest to a user, whereby said user may use said third message digest to navigate backward through said content space formed by said first and second computer files.