Klaus Ambos-Spies Problems with Cannot Be Reduced to Any Proper Subproblems. 162-168 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2747&spage=162 2003 conf/mfcs/2003 MFCS db/conf/mfcs/mfcs2003.html#Ambos-Spies03