  The complexity classes Unique End of Potential Line (UEOPL) and its promise
version PUEOPL were introduced in 2018 by Fearnly et al. UEOPL captures search
problems where the instances are promised to have a unique solution. UEOPL
captures total search versions of these promise problems. The promise problems
can be made total by defining violations that are returned as a short
certificate of an unfulfilled promise.
  GridUSO is the problem of finding the sink in a grid with a unique sink
orientation. It was introduced by G\"artner et al. We describe a promise
preserving reduction from GridUSO to UniqueForwardEOPL, a UEOPL-complete
problem. Thus, we show that GridUSO is in UEOPL and its promise version is in
PUEOPL.
