  Autonomous systems that operate in a shared environment with people need to
be able to follow the rules of the society they occupy. While laws are unique
for one society, different people and institutions may use different rules to
guide their conduct. We study the problem of reaching a common ground among
possibly incoherent rules of conduct. We formally define a notion of common
ground and discuss the main properties of this notion. Then, we identify three
sufficient conditions on the class of Horn expressions for which common grounds
are guaranteed to exist. We provide a polynomial time algorithm that computes
common grounds, under these conditions. We also show that if any of the three
conditions is removed then common grounds for the resulting (larger) class may
not exist.
