antiunification
Functional implementation of anti-unification algorithm for multiple terms
Adapted from figure 2 of 'A functional reconstruction of anti-unification' by Bjarte M. Østvold Norwegian Computing Center DART/04/04 2004
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.95.9108&rep=rep1&type=pdf