code: public EntityMapping addSource(EntityType source) {
      if (entityMappings.containsKey(source.getId())) {
        throw new IllegalStateException("Mapping already present for source " + source.getId());
      }
      EntityMapping result = new EntityMapping(source, target);
      entityMappings.put(source.getId(), result);
      return result;
    }
docs: Adds a new {@link EntityMapping} to this target for a certain source.
  
  @param source {@link EntityType} for the source entity that is mapped to this target
  @return the newly created empty {@link EntityMapping}