code: public void addPrimaryKey(StorableInfo<S> info) {
          if (info == null) {
              throw new IllegalArgumentException();
          }
          addKey(info.getPrimaryKey());
      }
docs: Adds the primary key of the given storable as indexes by calling {@link
  #addKey addKey}. This method should not be called if the primary key
  cannot be altered because persistent data is already stored against
  it. Instead, the primary key index should be added as a normal index.
  
  <p>After adding the primary key via this method and after reducing the
  set, call {@link #findPrimaryKeyIndex findPrimaryKeyIndex} to get the
  best index to represent the primary key.
  
  @throws IllegalArgumentException if info is null