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answered 2016-09-07 20:33:01 +0100
There are so-called wild-cards, for exactly this purpose:
sage: test2 = arcsinh(1/3*x + 1/3) sage: w0 = SR.wild(0) sage: test2.substitute(arcsinh(w0) == log(w0)) log(1/3*x + 1/3)