code: def edges(self, tail_head_iter):
          
          edge = self._edge_plain
          quote = self._quote_edge
          lines = (edge % (quote(t), quote(h)) for t, h in tail_head_iter)
          self.body.extend(lines)
docs: Create a bunch of edges.
  
          Args:
              tail_head_iter: Iterable of ``(tail_name, head_name)`` pairs.