private boolean isMarked(Shape s) {
        return s.hasTrait(EventHeaderTrait.ID) || s.hasTrait(EventPayloadTrait.ID);
    }
Ensures that when an event structure contains an eventPayload member,
that all other members are bound to headers.

<p>Only a single member can be marked with the eventPayload trait, and
this is validated using {@link ExclusiveStructureMemberTraitValidator}.
smithy-model/src/main/java/software/amazon/smithy/model/validation/validators/EventPayloadTraitValidator.java