The option variable trigsign controls the application of the
reflection rule to erf. The user documentation doesn't mention
this. I was going to fix SF bug 1860250. Unless somebody can
convince me otherwise, I'll keep the undocumented trigsign
flag in simperf. Maybe somebody would like to do build an coherent
scheme for controlling reflection simplifications.
(%i2) erf(-x), trigsign : true;
(%o2) -erf(x)
(%i3) erf(-x), trigsign : false;
(%o3) erf(-x)
Barton