BGMPRG(A) was renamed to WAVSET(A) sometime during the development of SB (they were named BGMPRG(A) in PTC), but the original functions were not removed. They appear to act identically.
It's not a bug either; thread should be reclassified, or...something.
Also, don't bump if you're not going to be constructive.
Apologies. I was trying to make a point. It was not the best way for me to do it. Thank you for confirming my point and making it explicit, though.
We've decided to put undocumented features and manual mistakes in this forum, since the categories overlap so much.