@ARTICLE {graded,
title = {Graded sparse graphs and matroids},
author = {Lee, Audrey and Streinu, Ileana and Theran, Louis},
journal = {Journal of Universal Computer Science},
year = {2007},
note = {\url{http://arxiv.org/abs/0711.2838}},
volume = {13},
number = {10}
}