@inproceedings{NFM2012:LiXu,
 author = {Xinxin Liu and Bingtian Xue},
 title = {Specification in PDL with Recursion},
 booktitle = {Proceedings of the 4th  NASA Formal Methods Symposium (NFM 2012)},
 volume = {7226},
 editor = {Alwyn E. Goodloe and Suzette Person},
 location = {Norfolk, VA, USA},
 publisher = {Springer-Verlag},
 address = {Berlin, Heidelberg},
 pages = {181--194},
 month = {April},
 year = {2012}
}