Reserve Mathematica function names and indicate they are unimplemented

[ghost]

In[3]:= Fit[{1,2,4,8,17},{1,x,x^2},x]
Out[3]= Fit[{1, 2, 4, 8, 17}, {1, x, x ^ 2}, x]

Instead of returning its input essentially unchanged, it should return some version of "We know the Fit function exists in Mathematica, but isn't currently implemented in Mathics. However, Fit is a reserved word, so you can not use it as a name for one of your own functions"

[wolfv]

Do you know if there is a reference list of all Mathematica functions somewhere?

[ghost]

https://reference.wolfram.com/language/guide/AlphabeticalListing.html

You can get also get these (+ some more stuff) by doing "?*" in Mathematica

Additionally, Mathics should disallow (or at least discourage) user functions where the first letter is capitalized, since Mathematica reserves these names for itself, and it may cause a conflict in the future.

Same for variables starting with '$'.