`A``=``1/2``-``2/3``+``3/4``-``4/5``+``5/6``-``6/7``-``5/6``+``4/5``-``3/4``+``2/3``-``1/2``
`=``(``1/2``-``2/3``+``2/3``-``1/2``)``+``(``3/4``-``4/5``+``4/5``-``3/4``)``+``(``5/6``-``5/6``)``-``6/7`
`=``0``+``0``+``0``-``6/7`
`=``-``6/7`
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