David A. Plaisted Special Cases and Substitutes for Rigid E-Unification. 97-152 2000 10 Appl. Algebra Eng. Commun. Comput. 2 http://link.springer.de/link/service/journals/00200/bibs/0010002/00100097.htm db/journals/aaecc/aaecc10.html#Plaisted00