anchor.diff:  
   The call to +configureOptionsFor()+, passing the store-specific identifier type +MongoDB+,
   provides the entry point into the API. Following the fluent API pattern, you then can configure
  -global options (+writeConcern()+) and navigate to single entities or properties to apply options
  +global options (+writeConcern()+, +readPreference()+) and navigate to single entities or properties to apply options
   specific to these (+associationStorage()+ etc.).
   The call to +writeConcern()+  for the +Animal+  entity shows how a specific write concern type can be used.
   Here +RequiringReplicaCountOf+ is a custom implementation of +WriteConcern+ which ensures
  
positive.code: When working with MongoDB, you can currently configure the following options using the API:
  
  * write concern
  * association storage strategy
  * association document storage strategy
  
  
negative.code: When working with the MongoDB backend, you can specify the following settings:
  
  * the write concern for entities and associations using the +@WriteConcern+ annotation
  * the read preference for entities and associations using the +@ReadPreference+ annotation
  * a strategy for storing associations using the +@AssociationStorage+ and +@AssociationDocumentStorage+ annotations
  (refer to <<ogm-mongodb-storage-prinicples>> to learn more about these options).
  
  