To give more context, B. Pierce, one of the authors of the Dropbox paper is (one of?) the main author of Unison, and also coauthor of the paper you linked.

EDIT: The parent cited http://www.cis.upenn.edu/~bcpierce/papers/unisonspec.pdf by Pierce in 2004, where the authors stressed the difficulty of writing a specification for a file synchronizer, in this case Unison.