ochre

Copperbox revision 2870.

I've added pure let - previously I just had effectful let, but it wasn't named as such. I've also added effectful let for 2,3 and 4 tuples. The introduction form is actually more like Scheme's let-values than proper tuples - Csound doesn't have tuples itself so I have to be very careful about their introduction and elimination in the embedded language.