Alfons Geser Aart Middeldorp Enno Ohlebusch Hans Zantema Relative Undecidability in Term Rewriting. 150-166 1996 conf/csl/1996 CSL db/conf/csl/csl96.html#GeserMOZ96 http://dx.doi.org/10.1007/3-540-63172-0_37