Evaluation order in DEFSKETCH
I tried this:

```
(DEFSKETCH xxx (...)
    ((x-left 1.0d0)
     (x-right 1.0d0)
     (x-step (/ (- x-right x-left) width))
...))
```

but that fails with "unbound variable". Can/should that be similar to `LET*`, perhaps?
