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Section 3 (Specification) of the proposed SRFI includes the paragraph
The expander maintains hygiene with the help of marks and substitutions. Marks are applied selectively by the expander to the output of each transformer it invokes, and substitutions are applied to the portions of each binding form that are supposed to be within the scope of the bound identifiers. Marks are used to distinguish like-named identifiers that are introduced at different times (either present in the source or introduced into the output of a particular transformer call), and substitutions are used to map identifiers to their expand-time values.
Should marks and substitutions be considered part of the specification, or an implementation technique? I would suggest that the SRFI author try to find a way to specify the macro system precisely without going into details of how the expander is implemented. David Feuer