#4690 inverse_erf(erf(x)) and erf(inverse_erf(x)) don't simplify to x, even if x is real

[Robert Dodier]

Working with a current version (c1b3689). I notice that erf(inverse_erf(x)) and inverse_erf(erf(x)) don't simplify to x, even after declare(x, real).

I see that both %ERF and %INVERSE_ERF have a property $INVERSE, which has the value %INVERSE_ERF and %ERF, respectively. I don't know how $INVERSE is supposed to work, but anyway it seems like the intent is that these inverses should be applied and they're not.

For complex x, inverse_erf(x) is multivalued. I don't know how Maxima generally handles stuff like that. I guess maybe we can arrange for the principal value to be returned. Maybe there is already code for this which isn't being called -- I haven't looked.