code: def _tarjan_head(ctx, v):
          
          ctx.index[v] = len(ctx.index)
          ctx.lowlink[v] = ctx.index[v]
          ctx.S.append(v)
          ctx.S_set.add(v)
          it = iter(ctx.g.get(v, ()))
          ctx.T.append((it,False,v,None))
docs: Used by @tarjan and @tarjan_iter.  This is the head of the
              main iteration