Isn't most of the value of Mathematica in the standard library instead of the language?

Yes, definitely! So we also try to implement as much of it as possible. Currently we support over 900 functions of the 6000 from Mathematica!

Call me when you have the upgoat/downgoat functions implemented :-)

Referring to https://codegolf.stackexchange.com/questions/71631/upgoat-or... I assume.

Reminder that open source is always open for contributions

Isn't there already Maxima which is several decades old?

Maxima, Axiom, SymPy, and many more CAS systems exist. They don't run Mathematica code, so if that's your goal they don't help.

Does that include symbolic math equivalent to that of Maxima?

Yes, definitely! This is one of Mathematica’s hallmark features and it would not be complete without it.