[dotancohen]

  > They put their "step-by-step" explanations behind a [login/pay]wall which made it significantly less useful.

Maybe, but what else can do step-by-step explanations? Perhaps octave?

[fisian]

SymPy Gamma, for example: https://www.sympygamma.com/input/?i=diff%28a**%285*x**2%29%2...