(indemnity action and question of whether there is a duty to defend under farm owner’s insurance policy where farmer used his tractor to move a colleague’s trailer home on public road and the actions resulted in significant damage to the trailer home and colleague’s vehicle after tractor stalled on hill and trailer home rolled backward into colleague’s personal vehicle; question involved the “four-corners rule” as to whether insurance policy covered the property damage; court holds when insurance company provides defense, purpose of four-corners rule is met; extrinsic evidence is relevant and can be presented in determining whether coverage exists; policy language ambiguous under the facts of the case, so to be interpreted in favor of coverage; questions of fact regarding whether trailer in care of the insured precludes entry of summary judgment ; court of appeals decision affirmed).