public static <T> Set<T> merge(Manager<T> manager, Set<T> entities) {
        return InternalMergeUtil.merge(manager, entities);
    }
Merges the provided {@code entities} with the underlying database.
<p>
The entity must have exactly one primary key.

@param <T> entity type
@param manager to use for merging
@param entities to merge with the database
@return the entities that have been merged
@throws UnsupportedOperationException if the entity class
        does not have exactly one primary key
runtime-parent/runtime-core/src/main/java/com/speedment/runtime/core/util/MergeUtil.java