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使いかた
http://pubman.mpdl.mpg.de/pubman/item/escidoc:150722:1/component/escidoc:150721/cadabra.pdf
A brief introduction to Cadabra: a tool for tensor computations in General Relativity
https://arxiv.org/pdf/0903.2085.pdf
https://suppiya.files.wordpress.com/2008/05/comprund_41_016.pdf