GDL newbie-ness corrected (oops)

[Spuzzum]

Just so you're all aware, that little (T#1 T#2) error of mine was caused by
the following line.

(set river-sink-terrain (shallows swamp))

I understand what's syntactically wrong with that.  It only accepts one
terrain type.  Which is why it was telling me, in its obfuscated little way
( ;-) ), that T#2 had to be a terrain type, not a list.


...In the future, I wonder if it could pick between sink terrains, like
that, by checking the occurrence variable as weighting?  Food for thought,
anyway...