Erroneous unification

[andrew-johnson-4]

(Frg(
   t tt
))

is accepted instead of

(Frg(
   k t tt
))

even though one field is missing

[andrew-johnson-4]

When constructed in set position

(set x (Abs( lt rt )))

bad stuff gets through.

In let position

(let x (Abs( lt rt )))

errors will be caught normally

[andrew-johnson-4]

This is pretty big:

(set x (close y))

vs 

(set x y)

doesn't seem to matter.