@4dahttp://wiki.call-cc.org/eggref/4/low-level-macros
>Unhygienic macros can not be implemented with syntax-rules, so we must use one of the low-level transformers for the implementation.
http://en.wikipedia.org/wiki/Hygienic_macro
>Syntax-rules is a high-level pattern matching facility that attempts to make macros easier to write. However, syntax-rules is not able to succinctly describe certain classes of macros and is insufficient to express other macro systems. Syntax-rules was described in the R4RS document in an appendix but not mandated. Later, R5RS adopted it as a standard macro facility.
http://www.cs.indiana.edu/scheme-repository/R4RS/r4rs_12.html
>The primary limitation of the pattern language is that it is thoroughly hygienic, and thus cannot express macros that bind identifiers implicitly.