In the paper "Recurrence relations for the number of labeled structures on a finite set" by Blatter and Specker the authors speak of univalent, zerovalent and multivalent points of a tree.
It seems to me that a univalent point is a leaf of the tree. But the term "zerovalent point" doesn't make sense to me. And there is also the notion of multivalent ("not univalent" seems natural).
I could not find a definition of these terms. Can anybody help me?