What is the "inherency" doctrine as it relates to a prior art reference?

Remember, all elements of a claim must be taught or suggested by a single prior art reference in under 35 U.S.C. 102 rejection.  Sometimes, however, a specific feature in a claim is not explicitly taught by a prior art reference, but that feature is nonetheless "inherent" in the device or process described by the prior art reference. For example, suppose an inventor claims a mousetrap requiring a "flexible" spring.  The prior art reference discloses a steel spring, but there is absolutely no mention that this steel spring is "flexible."  Still, the spring described might be inherently flexible because of its materials and its design. If this were the case, the "flexible" element of the inventor's claim would be met by the reference, even though "flexible" was never mentioned explicitly.