Thanks again Philip for taking the time to reply.

This is a rather unpleasant pitfall of the REPL. If you try to evaluate the 
> expression `(if #f some-unbound-identifier 1)`, you will see that it 
> evaluates to `#f` in the REPL but raises an unbound identifier error in a 
> module. At the REPL, `some-unbound-identifier` refers to a top-level 
> variable, and it is allowed to support, for example, forward references to 
> identifiers that will be defined in a subsequent interaction, or 
> interactive re-definition of variables.

When entering '(hex a b c 1 2 3)' into the REPL, I don't think the symbols 
'a', 'b' and 'c' are undefined or forward-references as they appear in the 
taken branch of the conditional. Maybe syntax objects are different coming 
from the REPL than a module, somehow resulting in the macro working as 

Anyway, I take your point the top level has issues. I will be far more wary 
of it in future.

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