def add_statement(key, obj)
      if obj.is_a?(RDF::Statement)
        @source[normalize_key(key)] = obj 
        local_repository << obj
      end
    end
def add_statement(key, obj)
      if obj.is_a?(RDF::Statement)
        @source[normalize_key(key)] = obj 
        local_repository << obj
      end
    end
Set RDF::Statements to the underlying collection.  Normalizes the keys.
 @param [String, Symbol] key
 @param [RDF::Statement] obj.  RDF::Statement that will be added.