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https://github.com/hiroshi0530/wa-src/blob/master/article/library/sympy 
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CustomError: Could not open notebook at https://api.github.com/repos/hiroshi0530/wa-src/git/trees/b1b77851a9f963544864768048e3c0b4adee0c69
    at new sM (https://ssl.gstatic.com/colaboratory-static/common/c3b8c7dca2abfd16eb021f6c96299a7e/external_binary.js:2450:68)
    at va.program_ (https://ssl.gstatic.com/colaboratory-static/common/c3b8c7dca2abfd16eb021f6c96299a7e/external_binary.js:2438:264)
    at xa (https://ssl.gstatic.com/colaboratory-static/common/c3b8c7dca2abfd16eb021f6c96299a7e/external_binary.js:16:57)
    at va.next_ (https://ssl.gstatic.com/colaboratory-static/common/c3b8c7dca2abfd16eb021f6c96299a7e/external_binary.js:14:195)
    at kaa.next (https://ssl.gstatic.com/colaboratory-static/common/c3b8c7dca2abfd16eb021f6c96299a7e/external_binary.js:17:48)
    at b (https://ssl.gstatic.com/colaboratory-static/common/c3b8c7dca2abfd16eb021f6c96299a7e/external_binary.js:17:309)
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