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This paper describes a modified form of Kohlbecker’s algorithm for reliably hygienic(capture-free) macro expansion in block-structured languages, where macros are source-tosource transformations specified using a high-level pattern language. Unlike previous algorithms, the modified algorithm runs in linear instead of quadratic time, copies few constants, does not assume that syntactic keywords(eg if) are reserved words, and allows 10CSJ (scoped) macros to refer to lexical variables in a referentially transparent manner. Syntactic closures have been advanced as an alternative to hygienic macro expansion. The problem with syntactic closures is that they are inherently low-level and therefore difficult to use correctly, especially when syntactic keywords are not reserved. It is impossible to construct a patternbased, automatically hygienic macro system on top of syntactic closures because the pattern interpreter must …