def each_vertex(filter=nil, &block)
      if filter.nil?
        @vertices.each &block
      else
        vertices(filter).each &block
      end
    end
def each_vertex(filter=nil, &block)
      if filter.nil?
        @vertices.each &block
      else
        vertices(filter).each &block
      end
    end
Calls block on each graph vertex for with the 'filter and block' predicate
 evaluates to true.