def edge_style(self, head, tail, **kwargs):
        '''
        Modifies an edge style to the dot representation.
        '''
        if tail not in self.nodes:
            raise GraphError("invalid node %s" % (tail,))

        try:
            if tail not in self.edges[head]:
                self.edges[head][tail]= {}
            self.edges[head][tail] = kwargs
        except KeyError:
            raise GraphError("invalid edge  %s -> %s " % (head, tail) )
Modifies an edge style to the dot representation.