Manfred Schmidt-Schauß Decidable Variants of Higher-Order Unification. 154-168 2005 conf/birthday/2005siekmann Mechanizing Mathematical Reasoning http://springerlink.metapress.com/openurl.asp?genre=article&id=KKKCHVDKPUQB1V51 db/conf/birthday/siekmann2005.html#Schmidt-Schauss05