You are approaching an oncoming vehicle in a narrowing of the road on an upward slope. If there is no signage to regulate right of way, who should go first?

Explanation
If two vehicles reach a narrowing of the road with a steep slope and no signage indicated right of way, where the road is too narrow for both vehicles to pass at the same time, right of way will always be given to the vehicle traveling uphill, except when they can get to a sidetrack designed to allow the downhill vehicle to pass first.
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