Makoto Hamana Free S-Monoids: A Higher-Order Syntax with Metavariables. 348-363 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3302&spage=348 2004 conf/aplas/2004 APLAS db/conf/aplas/aplas2004.html#Hamana04