To discover the line of the error the interpreter should provide a way to associate every form or symbol passed to the macro with its line number. As an example there could be a function such as (line-num form-with-an-error) to get the line number.
My "attachments" idea was conceived with this potential use too - jus attach the line number to the object, which acts just like the original object (i.e. matches with 'is, etc.).
