weird bug (& implementing monad transformers with units)

[Daniel Mahler <mahler@cyc.com>]

Hi,

I started having a go at developing a monad transformer framework
using units for what is noramlly done with type classes and functors
ind haskell/ml respectively.

I started getting the following bug
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Welcome to DrScheme, version 103.
Language: Graphical Full Scheme (MrEd).
internal-syntax-error: compound-unit: duplicate external name at: amb in: (#%compound-unit (import) (link (n ((#%with-continuation-mark (#%quote debug-key-127) #<struct:zodiac> ((#%with-continuation-mark (#%quote debug-key-127) #<struct:zodiac> #%unit-with-signature-unit) n)) (l amb) (l bind) (l const) (l fmap) (l join)...
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

My code is attached below.
This is my first go at using units so I am also
interested in any comments/advice on style.
(I seem to be getting more redundant looking code then I expected.)

regards
Daniel Mahler

monads.scm