code: public static synchronized EntityManagerProvider instance(String unitName, Map<String,String> overridingPersistenceProps) {
          overridingProperties = overridingPersistenceProps;
          return instance(unitName);
      }
docs: Allows to pass in overriding Properties that may be specific to the JPA Vendor.
  
  @param unitName unit name
  @param overridingPersistenceProps properties to override persistence.xml props or define additions to them
  
  @return EntityManagerProvider instance