Does an umbrella policy need to prorate or otherwise contribute concurrently with a primary policy?

We are therefore called upon to determine whether an umbrella policy, providing only excess insurance for the occurrence involved here, must prorate or otherwise contribute concurrently with a primary policy which, by operation of a contingency clause, in essence becomes an excess policy. In doing so, we are mindful of the basic rule that in interpreting policies of insurance, "[t]he policy will be viewed in light of its general objects and purposes." (Otter v. General Ins. Co. (1973) 34 Cal.App.3d 940, 949, 109 Cal.Rptr. 831.)